ci: lint PR titles with commitlint (#37498)

## Summary
- Enforce **Conventional Commits** on PR titles (PRs are squash-merged,
so the PR title becomes the final commit message).
- Add a local `make lint-pr-title` target so contributors can validate
titles before pushing.

## Why
We squash-merge PRs, which means the final repository history is largely
shaped by **PR titles**. Enforcing a consistent Conventional Commits
format makes:
- **Release notes & changelogs easier to generate** (types like `feat` /
`fix` can be grouped automatically).
- **History easier to scan** (uniform structure, optional scopes,
explicit breaking changes via `!`).
- **Automation more reliable** (future tooling can infer category and
scope from the title).

## PR title format
```text
type(scope)!: subject
type: one of build|chore|ci|docs|feat|fix|perf|refactor|revert|style|test
scope: optional (e.g. web, api, actions, repo, …)
!: optional, indicates a breaking change
subject: short, imperative, no trailing period
```

## Examples
```text
feat(web): add dark mode toggle
fix(api): avoid panic when repo is missing
chore(ci): lint PR titles with commitlint
refactor(templates): reduce duplication in repo list rendering
feat!: remove legacy OAuth endpoint
```

## Local testing
```text
make deps-frontend
make lint-pr-title PR_TITLE="feat(web): add dark mode toggle"
```

---------

Signed-off-by: Nicolas <bircni@icloud.com>
Co-authored-by: nb <nb@users.noreply.local>
Co-authored-by: GPT-5.2 <gpt-5.2@openai.com>

AGENTS.md

@@ -7,6 +7,7 @@
 - Run single playwright e2e test files with `GITEA_TEST_E2E_FLAGS='<filepath>' make test-e2e`
 - Add the current year into the copyright header of new `.go` files
 - Ensure no trailing whitespace in edited files
+- Use Conventional Commits format for commit messages and PR titles (e.g. `type(scope): subject`)
 - Never force-push, amend, or squash unless asked. Use new commits and normal push for pull request updates
 - Preserve existing code comments, do not remove or rewrite comments that are still relevant
 - In TypeScript, use `!` (non-null assertion) instead of `?.`/`??` when a value is known to always exist