fix(site/src/pages/AgentsPage): use the root chat's project for subagent chat skills - #30840

Adds the storage and authorization layer for chat project skills. The
`skills` table already allows a `project_id` owner (migration 000621);
this adds the queries for it and authorizes them through the parent chat
project, the same way project instructions work.

- New queries in `skills.sql`: insert, get by name, list metadata,
update (COALESCE on description, content and enabled), delete, and
`GetProjectSkillByIDForUpdate` for the shared update lock.
- dbauthz checks `chat_project:read` on the project for reads and
`chat_project:update` for writes. `authorizeChatProjectInstructions`
became `authorizeChatProjectConfig`, shared by instructions and skills.
`Skill.RBACObject()` still denies project rows so nothing reaches them
without going through the project.
- Audit: new `chat_project_skill` resource type (enum-only migration
000624), `AuditableChatProjectSkill`, and the usual
diff/request/table/FriendlyString/AuditFilter wiring. The API that emits
these entries comes in the next PR.

Stack (merge top to bottom):
- #30826 👈
- #30827
- #30838
- #30840
- #30854
- #30855
- #30856
- #30861
- #30880
- #30881
- #30882
- #30884

Adds experimental routes for managing a chat project's skills under
`/api/experimental/organizations/{organization}/chats/projects/{project}/skills`:
list, create, get, update (PATCH) and delete by skill name.

The handlers reuse the shared skill create and update path from the
personal and organization skill APIs. Reading needs `chat_project:read`
on the project and writing needs `chat_project:update`; callers without
access get 404, matching the project instructions routes. Duplicate
names and the 100-skill-per-project cap return 409. Writes are audited
as `chat_project_skill` with the skill name as the target. The SDK gains
the matching `ExperimentalClient` methods.

Stack (merge top to bottom):
- #30826
- #30827 👈
- #30838
- #30840
- #30854
- #30855
- #30856
- #30861
- #30880
- #30881
- #30882
- #30884