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/ui/app/app_test.go

@@ -973,16 +973,16 @@ func TestSourcesFavouritesTab(t *testing.T) {
 func TestRebindingShowsEverywhere(t *testing.T) {
 	t.Parallel()
 	km := keys.Default()
-	if err := km.Apply(map[string]map[string][]string{"lists": {"reveal": {"e"}, "versions": {}}}); err != nil {
+	if err := km.Apply(map[string]map[string][]string{"lists": {"reveal": {"z"}, "versions": {}}}); err != nil {
 		t.Fatal(err)
 	}
 	d := newDriverOpts(t, Options{Keys: &km})
 	d.keys("3")
 	scr := d.screen()
-	if !strings.Contains(scr, "(e to reveal)") || strings.Contains(scr, "space to reveal") {
+	if !strings.Contains(scr, "(z to reveal)") || strings.Contains(scr, "space to reveal") {
 		t.Errorf("detail hint doesn't follow the binding:\n%s", scr)
 	}
-	if !strings.Contains(lastLine(scr), "e reveal/hide") {
+	if !strings.Contains(lastLine(scr), "z reveal/hide") {
 		t.Errorf("help bar: %q", lastLine(scr))
 	}
 	if strings.Contains(scr, "to browse") {
@@ -992,9 +992,9 @@ func TestRebindingShowsEverywhere(t *testing.T) {
 	if strings.Contains(d.screen(), "correct-horse") {
 		t.Error("space still reveals")
 	}
-	d.keys("e")
+	d.keys("z")
 	if !strings.Contains(d.screen(), "correct-horse-battery-staple") {
-		t.Errorf("e doesn't reveal:\n%s", d.screen())
+		t.Errorf("z doesn't reveal:\n%s", d.screen())
 	}
 }