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>

CONTRIBUTING.md

@@ -189,6 +189,22 @@ In the PR title, describe the problem you are fixing, not how you are fixing it.
 Use the first comment as a summary of your PR. \
 In the PR summary, you can describe exactly how you are fixing this problem.
 
+PR titles must follow the [Conventional Commits](https://www.conventionalcommits.org/) format, because PRs are squash-merged and the PR title becomes the resulting commit message:
+
+```text
+type(scope)!: subject
+```
+
+The allowed types are `build`, `ci`, `docs`, `feat`, `fix`, `perf`, `refactor`, `revert`, `style`, and `test`. The generic `chore` type is intentionally not accepted; pick a more descriptive type instead.
+
+Examples:
+
+```text
+fix(web): prevent avatar upload crash on empty file
+feat(api): add pagination to repo hooks list
+ci(workflows): lint PR titles with commitlint
+```
+
 Keep this summary up-to-date as the PR evolves. \
 If your PR changes the UI, you must add **after** screenshots in the PR summary. \
 If you are not implementing a new feature, you should also post **before** screenshots for comparison.