feat: share env files as encrypted links (#54)

Share `.env` files as an encrypted `lazysecrets://share/…` link. The files are encrypted with a secret's value, so anyone who can read that secret can open the link.

**Sharing:** `S` → `f` on a repo (all its plain env files), an env file, or a key in one (its file).
- **The key** is `share.key` from the repo's `.lazysecrets.yaml`, else from the config. It can be a `lazysecrets://` link, a Key Vault URL or an `@Microsoft.KeyVault(...)` reference.
- **Without `share.key`,** a picker built on Go to opens, titled "Share .env", with "Pick a secret to encrypt with" in its own border. It lists only secrets, leaving out values from plain `.env` files. Pasting a reference picks it. Picking shares at once, and the picker stays open until the link is copied. If the secret can't be used, the reason shows in red in the picker: it can't be read, or it doesn't meet `share.keyRules` (which rule failed is named). `esc` cancels.
- **`share.keyRules`** (defaults `minLength: 32`, `minClasses: 2`; never under 16) applies when sharing, not when opening.
- **On Azure,** the link pins the key's version, so rotating the key doesn't break old links, and disabling a version revokes them.
- **The command line:** `lazysecrets share [--key <secret>] <env files>`.

**Opening** works by paste, `ctrl+o`, the clipboard offer, the link handler or `lazysecrets <link>`.
1. It first says which repo the link is from, how many files it holds and which key it uses, and decrypts only after `o`.
2. It saves into the repo's clone, found by its remote, showing which files are new, which would replace one there, and which git doesn't ignore.
3. Without a clone, it asks for a folder and saves straight away. The prompt stays open, with the reason in red, until the files are written.
4. If decryption fails, a popup says why: the first line of the reason, `r` for the whole message, `g` to go to the key's secret. A key that has changed is named by where it lives: a Databricks or SOPS value changed in place, an Azure current version that isn't the one the link was made with, or a pinned version, meaning the link may be damaged.

**Format** (`internal/share`): a version byte, then a JSON header with the key, the repo and the file count, then an age file (scrypt 2^15, with the key's value as passphrase) holding the files as deflated JSON, all as unpadded base64url. Links are capped at 16 KB. A future format gets a new version, and old ones stay readable.

**Safety:**
- A link can only write env files, inside the folder: never in `.git`, never through a symlink.
- New files are created with mode 0600.
- Opening accepts an scrypt work factor of at most 2^20, and unpacks at most 1 MB.

**Along the way:**
- Opening a link to a secret ends on its details, as `enter` would.
- `o` on an Azure secret opens its portal page, and in the versions view, that version's page.
- The details pane follows its secret's list when it reloads in the background. Before, it kept what it showed first, including an old portal address.
- `ref.Read` is shared by `get` and sharing, with Azure `VersionedValue` for pinning.
- `provider.Contents` lets the demo repos share their env files. `--demo` has a share key (`kv-shared-prod / env-share`), so the whole flow works there.
- New dependency: `filippo.io/age`.

Tested end to end in the TUI and the demo, the CLI, and the `share` package. In a real vault: version pinning, opening into a clone, and revocation by disabling the version.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #54

docs/links.md

@@ -1,9 +1,10 @@
 # Links
 
-Every subscription, workspace, vault, scope, repo, file and secret has a
-`lazysecrets://` link. Share one in chat or a ticket, and whoever opens it
-lands on that item, reading it with their own account and permissions:
-links carry names, never values.
+Subscriptions, workspaces, vaults, scopes, repos, files and secrets can
+be shared as a `lazysecrets://` link, in chat or a ticket. Whoever opens
+it lands on that item, reading it with their own account and
+permissions: the link names the item, never its value. Env files can
+also be [shared encrypted](#sharing-env-files), in the link itself.
 
 ```
 lazysecrets://azure/kv-app-prod.vault.azure.net                              vault
@@ -20,10 +21,83 @@ lazysecrets://repo/git.example.com%2Fteam%2Finfra/secrets%2Fapp.yaml/db.password
 that version). `S` opens the share menu:
 
 - secrets: the link (`o`), the plain Key Vault URL (`u`), the config
-  reference (`y`) or the web page (`b`)
+  reference (`y`) or the web page (`b`); in an env file, also the file as
+  an [encrypted link](#sharing-env-files) (`f`)
 - vaults and scopes: a link (`o`), a link that opens on the current tab
   (`t`), the plain vault URL (`u`) or the web page (`b`)
 - subscriptions and workspaces: a link (`o`) or the web page (`b`)
+- repos and env files: also their env files as an
+  [encrypted link](#sharing-env-files) (`f`)
+
+## Sharing env files
+
+`.env` files usually live only on your machine. `S` → `f` shares them as
+an encrypted link, on a repo (all its plain env files), an env file, or a
+key in one (its file):
+
+```
+lazysecrets://share/AQ…
+```
+
+The files are encrypted with a secret's value, so whoever can read that
+secret (in Key Vault, Databricks or a repo's SOPS file) can open the link,
+and nobody else: who may open it is up to the vault's access, as for
+everything else. Make a secret for it once, random:
+
+```sh
+az keyvault secret set --vault-name kv-team --name env-share --value "$(openssl rand -base64 32)"
+```
+
+and name it in the repo's `.lazysecrets.yaml` (committed: the whole team
+uses it) or your config (for repos that don't name one), by its
+`lazysecrets://` link, Key Vault URL or `@Microsoft.KeyVault(...)`
+reference:
+
+```yaml
+share:
+  key: https://kv-team.vault.azure.net/secrets/env-share
+```
+
+With neither, sharing asks which secret, in [Go to](keys.md#go-to): it
+lists only secrets, and `enter` picks one (pasting a reference picks it
+too).
+
+Anyone with a link can try to guess the key, so sharing refuses one that
+isn't long and varied enough, saying how (never showing it). By default
+that's 32 characters of two kinds; your config can ask for more, or less:
+
+```yaml
+share:
+  keyRules:
+    minLength: 32    # characters; 16 at least
+    minClasses: 2    # kinds of character: lowercase, uppercase, digits, symbols
+```
+
+Opening a link doesn't check them.
+
+On the command line, `lazysecrets share [--key <secret>] <env file>...`
+prints the link.
+
+Opening a share link says which repo it's from, how many files and which
+secret it's encrypted with, and decrypts only when asked: lazysecrets
+reads the secret with your access, then shows where each file goes in
+your clone of the repo (or asks for a folder, if it isn't here), which
+ones would replace a file there and which ones git doesn't ignore. It
+saves them all, only the new ones, or none. New files are readable by you
+only.
+
+A link holds the files' paths and contents encrypted ([age](https://age-encryption.org),
+the secret's value as passphrase); the repo, the secret and the number of
+files are in the clear. Links stay under 16 KB, which fits chat and
+tickets.
+
+A link stays readable as long as its secret is. On Azure it's pinned to
+the secret's version, so adding a new version (rotating) doesn't break
+old links, and disabling the old version, or taking away someone's
+access, shuts them out; Databricks secrets have no versions, so changing
+the value breaks every old link. Whoever has read the secret's value
+before can decrypt links made with it without asking the vault, so rotate
+it now and then: new links use the newest version.
 
 ## Opening
 
@@ -44,9 +118,9 @@ read the clipboard over SSH.
 
 lazysecrets finds the vault in your accounts (one Resource Graph query per
 account) and jumps to it, asking which account to use when several can see
-it. Links say what, not what to do: opening a link to a secret asks whether
-to just go to it, also copy or show its value, or (`y`) copy its value
-without going there.
+it, ending on a secret's details as `enter` on it would. Links say what,
+not what to do: opening a link to a secret asks whether to just go to it,
+also copy or show its value, or (`y`) copy its value without going there.
 
 ## In scripts