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>

tools/lint-pr-title.js

@@ -0,0 +1,19 @@
+#!/usr/bin/env node
+import {env, exit} from 'node:process';
+
+const allowedTypes = 'build, ci, docs, feat, fix, perf, refactor, revert, style, test';
+const title = env.PR_TITLE;
+
+if (!title) {
+  console.error('Missing PR_TITLE');
+  exit(1);
+}
+
+const validTitlePattern = new RegExp(`^(${allowedTypes.replaceAll(', ', '|')})(\\([\\w.-]+\\))?(!)?: .+$`);
+
+if (!validTitlePattern.test(title)) {
+  console.error(`Invalid PR title: ${title}`);
+  console.error('Expected format: type(scope): subject');
+  console.error(`Allowed types: ${allowedTypes}`);
+  exit(1);
+}