feat: e opens a repo's SOPS or .env file, in nvim, sops edit or your editor (#1)

`e` on a file in a repo opens it; on a secret, it opens the file at that key's line.

- **SOPS files**: `sops edit` in the terminal, then back to lazysecrets with the keys reloaded. Exit 200 (nothing changed) shows "unchanged".
- **Other files inside nvim** (`$NVIM` set): lazysecrets asks that nvim, over `--remote-expr`, to open the file once lazysecrets' terminal closes, then quits. The file lands in the window you came from, not the closing float. Started from a shell in a terminal, it opens in the previous window straight away. No change to the nvim plugin is needed.
- **Otherwise**: `$VISUAL`/`$EDITOR` in the terminal (with `+N`, `file:N` or `-g file:N` for editors that take a line), else the system default app.

The repo provider now records each key's line (YAML node lines, env/ini line numbers) behind a new optional `provider.Files` interface. Other providers show "only SOPS and .env files in repos can be opened".

The nvim handoff was tested against a headless nvim 0.12: a floating terminal closed on exit the way snacks does, and a shell in a split. A file name with a space and a quote opened at the right line in both. Unit tests cover lines per format, the nvim, sops and default-app paths, and the editor line arguments.

Note: `e` is a new default key, so a config that already binds `e` to something else now gets the "bound to both" error, until one of them is rebound.

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-authored-by: Karl-Einar Bergstrøm <karl-einarb@outlook.com>
Reviewed-on: #1

internal/provider/provider.go

@@ -169,3 +169,18 @@ type Current interface {
 type Locator interface {
 	Locate(ctx context.Context, address string) (sourceID string, ok bool, err error)
 }
+
+// Files is implemented by providers whose containers are files on disk
+// (SOPS and .env files in repos), so they can be opened in an editor.
+type Files interface {
+	// File is where c is, and for o (nil for the whole file) the line its
+	// key is on.
+	File(c Container, o *Object) (File, error)
+}
+
+// File is a container's file, to open.
+type File struct {
+	Path string
+	Line int    // 1-based; 0 for the top
+	Sops string // the sops command to edit it with; empty for a plain file
+}