feat: link each changelog entry to its PR and commit (#11)

Like Immich's and RomM's release notes: every changelog entry now links to its pull request and its commit, in `CHANGELOG.md` and in the release notes on Gitea:

```
- Search with fzf's scoring and query syntax ([#11](…/pulls/11)) ([65dc2d7](…/commit/65dc2d7…))
```

- `cliff.toml`: the entry template adds the short commit hash as a link; a postprocessor turns the squash commit's `(#N)` into a link to the PR.
- The app shows release notes as text (`U`, and the status pane's changelog to come), so `Release.Notes()` turns Markdown links back into their text: `Search (#11) (65dc2d7)`.

Checked with `git cliff --unreleased` on main: every entry since v0.1.0 gets both links. Test for the link stripping.

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

---------

Co-authored-by: Karl-Einar Bergstrøm <karl-einarb@outlook.com>
Reviewed-on: #11

internal/ui/app/update_test.go

@@ -47,8 +47,8 @@ func TestUpdate(t *testing.T) {
 	if f.asked != 1 {
 		t.Fatalf("asked %d times at startup", f.asked)
 	}
-	if !strings.Contains(d.screen(), "v0.1.1 ↑ v0.2.0 U") {
-		t.Errorf("no update in the footer:\n%s", d.screen())
+	if scr := d.screen(); !strings.Contains(scr, "[0] lazysecrets v0.1.1") || !strings.Contains(scr, "↑ v0.2.0 U") {
+		t.Errorf("no update in the status pane:\n%s", scr)
 	}
 
 	// U: the changelog, without the install notes; u installs.