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

internal/ui/app/links.go

@@ -46,6 +46,9 @@ type nav struct {
 	// quiet gives up without a word: going back to where lazysecrets
 	// was left, which may be gone.
 	quiet bool
+	// link ends on the object's details, as enter on it would: opening
+	// a link to it opens it, rather than just finding it in its list.
+	link bool
 }
 
 type resolvedMsg struct {
@@ -78,6 +81,9 @@ const (
 // just open it.
 // With ask false, vault and scope links open straight away.
 func (m *Model) offerRef(title string, r ref.Ref, ask bool) tea.Cmd {
+	if r.IsShare() {
+		return m.offerShare(r)
+	}
 	if r.IsContainer() || r.IsSource() {
 		if !ask {
 			return m.openRef(r, actGo)
@@ -398,23 +404,30 @@ func (m *Model) handleResolved(msg resolvedMsg) {
 		for i := range msg.choices {
 			msg.choices[i].label = "Open in " + msg.choices[i].label
 		}
-		m.chooseSource("Scope "+msg.ref.Scope+" is in several workspaces", msg.ref, msg.act, msg.choices)
+		m.chooseSource("Scope "+msg.ref.Scope+" is in several workspaces", msg.ref, msg.act, msg.choices, true)
 	case len(msg.others) > 0:
 		var choices []choice
 		for _, row := range append([]string{msg.sourceRow}, msg.others...) {
 			choices = append(choices, choice{label: "Open with " + m.sourceLabel(row), sourceRow: row})
 		}
-		m.chooseSource("Several accounts can see "+msg.ref.ContainerName(), msg.ref, msg.act, choices)
+		m.chooseSource("Several accounts can see "+msg.ref.ContainerName(), msg.ref, msg.act, choices, true)
 	case msg.linked != nil:
 		m.addLinked(*msg.linked)
-		m.startNav(msg.ref, msg.act, linksRowID)
+		m.startLinkNav(msg.ref, msg.act, linksRowID)
 	default:
-		m.startNav(msg.ref, msg.act, msg.sourceRow)
+		m.startLinkNav(msg.ref, msg.act, msg.sourceRow)
 	}
 }
 
+// startLinkNav opens a link: like startNav, ending on its object's
+// details.
+func (m *Model) startLinkNav(r ref.Ref, act linkAction, sourceRow string) {
+	m.startNav(r, act, sourceRow)
+	m.nav.link = true
+}
+
 // chooseSource asks where to open r.
-func (m *Model) chooseSource(title string, r ref.Ref, act linkAction, choices []choice) {
+func (m *Model) chooseSource(title string, r ref.Ref, act linkAction, choices []choice, link bool) {
 	var opts []option
 	for i, c := range choices {
 		key := ""
@@ -424,6 +437,7 @@ func (m *Model) chooseSource(title string, r ref.Ref, act linkAction, choices []
 		row := c.sourceRow
 		opts = append(opts, option{key: key, label: c.label, run: func(m *Model) tea.Cmd {
 			m.startNav(r, act, row)
+			m.nav.link = link
 			return nil
 		}})
 	}
@@ -574,6 +588,9 @@ func (m *Model) advanceNav() bool {
 				m.pendingCmd = m.toggleReveal()
 			}
 		}
+		if n.link {
+			m.focus = paneDetail // after: copying acts on the focused pane
+		}
 		return true
 	}
 	return false
@@ -607,6 +624,9 @@ func (m *Model) share(menu bool) tea.Cmd {
 		if s.URL != "" {
 			opts = append(opts, option{key: "b", label: webLabel(s.Kind), run: share("b")})
 		}
+		if m.canShareFiles(s) {
+			opts = append(opts, option{key: "f", label: "Encrypted link to its env files", run: func(m *Model) tea.Cmd { return m.shareFiles(s, nil) }})
+		}
 		m.showMenu("Share "+s.Name, nil, opts)
 	case paneContainers:
 		c, ok := m.containers.Selected()
@@ -629,6 +649,11 @@ func (m *Model) share(menu bool) tea.Cmd {
 		if c.URL != "" {
 			opts = append(opts, option{key: "b", label: webLabel(c.Source.Kind), run: share("b")})
 		}
+		if c.Detail == provider.DetailPlainEnv && m.canShareFiles(c.Source) {
+			opts = append(opts, option{key: "f", label: "Encrypted link to the file", run: func(m *Model) tea.Cmd {
+				return m.shareFiles(c.Source, []provider.Container{c})
+			}})
+		}
 		m.showMenu("Share "+c.Name, nil, opts)
 	default:
 		o, ok := m.detail.Secret()
@@ -653,6 +678,11 @@ func (m *Model) share(menu bool) tea.Cmd {
 		if o.URL != "" {
 			opts = append(opts, option{key: "b", label: webLabel(o.Container.Source.Kind), run: share("b")})
 		}
+		if c := o.Container; c.Detail == provider.DetailPlainEnv && m.canShareFiles(c.Source) {
+			opts = append(opts, option{key: "f", label: "Encrypted link to its file", run: func(m *Model) tea.Cmd {
+				return m.shareFiles(c.Source, []provider.Container{c})
+			}})
+		}
 		m.showMenu("Share "+o.Name, nil, opts)
 	}
 	return nil
@@ -802,6 +832,9 @@ func (m *Model) openText(s string) tea.Cmd {
 
 // handlePaste opens a pasted reference, unless a text input has focus.
 func (m *Model) handlePaste(msg tea.PasteMsg) tea.Cmd {
+	if m.search != nil && m.search.picker != nil {
+		return m.pickPasted(msg.Content) // a reference to the secret
+	}
 	if p := m.prompt; p != nil && p.input != nil {
 		in, cmd := p.input.Update(msg)
 		p.input = &in