ci(conventions): define title and commit vocabulary

[LMLiam]

Outcome

Pull-request titles and commit subjects use one documented verb and area vocabulary policy, independent of the validator implementation in #342.

Decision scope

Choose and document one of these policies before implementation:

1. Enforce reviewed verb and area sets, with one source of truth and an explicit extension process.
2. Keep verb and area values open, and define how every unrecognised release type appears in the generated changelog.

The current repository only checks lower-case syntax. It also contains historical areas that are more precise than module names. Do not silently convert the current recommendations into a restrictive allowlist.

Required evidence

• Inventory current commit subjects, pull-request titles, modules, feature packages, and explicit repository areas.
• Verify Release Please behaviour for every permitted verb.
• Treat settings and project metadata as the source for module names if refactor(meta): centralise module facts and enforce architecture boundaries #378 proves that they are suitable.

Non-goals

• Do not adopt the external action from ci(conventions): Adopt shared conventional validation action #342.
• Do not rewrite historical commits.
• Do not maintain different policies for pull-request titles and commit subjects.

Acceptance criteria

• One documented policy covers verbs and areas.
• Accepted module, feature, and repository areas have fixtures.
• Mistyped values and obsolete areas have fixtures when the policy rejects them.
• Every accepted verb has explicit Release Please behaviour.
• An extension changes the policy source, tests, and documentation together.