ci: add conventional commit PR title linting (#23096)

Restore PR title validation that was removed in 828f33a when
cdr-bot was expected to handle it. That bot has since been disabled.

The new title job in contrib.yaml validates:
- Conventional commit format (type(scope): description)
- Type from the same set used by release notes generation
- Scope validity derived from the changed files in the PR diff
- All changed files fall under the declared scope

Uses actions/github-script (no third-party marketplace actions).

Also fixes feat(api) examples across docs (no api folder exists)
and consolidates commit rules into CONTRIBUTING.md as the single
source of truth.

docs/about/contributing/CONTRIBUTING.md

@@ -247,8 +247,13 @@ characters long (no more than 72).
 
 Examples:
 
-- Good: `feat(api): add feature X`
-- Bad: `feat(api): added feature X` (past tense)
+- Good: `feat(coderd): add feature X`
+- Bad: `feat(coderd): added feature X` (past tense)
+
+Scopes must reference a real path in the repository (a directory or file stem)
+and must contain all changed files. For example, use `coderd/database` if all
+changes are within that directory. If changes span multiple top-level
+directories, omit the scope.
 
 A good rule of thumb for writing good commit messages is to recite:
 [If applied, this commit will ...](https://reflectoring.io/meaningful-commit-messages/).
@@ -263,7 +268,7 @@ to use the original commit title instead of the PR title.
 Breaking changes can be triggered in two ways:
 
 - Add `!` to the commit message title, e.g.
-  `feat(api)!: remove deprecated endpoint /test`
+  `feat(coderd)!: remove deprecated endpoint /test`
 - Add the
   [`release/breaking`](https://github.com/coder/coder/issues?q=sort%3Aupdated-desc+label%3Arelease%2Fbreaking)
   label to a PR that has, or will be, merged into `main`.