feat: add personal skill storage, API, and SDK (#25363)

> Mux updated this PR on behalf of Mike.

## Stack Context

This PR is the storage, permissions, API, and SDK layer for experimental
personal skills. #25362 has landed on `main`, so this branch is
restacked directly on `main`.

Stack order:
1. #25363 storage, permissions, API, and SDK
2. #25365 API test coverage
3. #25366 chattool and chatd integration
4. #25066 settings UI and docs
5. #25386 personal skills slash menu

## What?

Adds the `user_skills` database table, generated queries, RBAC resources
and scopes, audit resource handling, experimental user-scoped CRUD
endpoints, SDK types, and generated API/site types.

Follow-up review and restack fixes:
- Enforce a bounded personal skill description in parser and database
constraints.
- Return `403 Forbidden` for unauthorized create and update attempts.
- Return explicit conflict responses when soft-deleted users are
targeted.
- Keep user admins out of personal skills, while site owners can read
and delete but not create or update.
- Document trigger-raised constraint names and keep schema constants
covered by tests.
- Reuse `UserSkillMetadata` in the full `UserSkill` SDK response type.
- Generate user skill IDs in Go instead of relying on a database
default.
- Rebase on latest `main` and renumber the user skills migration to
`000502_user_skills`.

## Why?

Personal skills need durable user-owned storage with owner
authorization, limited site-owner moderation, and a hidden API surface
before chatd can consume them.

## Validation

- `make gen`
- `go test ./coderd/database -run '^TestUserSkillSchemaConstants$'
-count=1`
- `go test ./coderd/database/dbauthz -run
'^TestMethodTestSuite/TestUserSkills$' -count=1`
- `go test ./coderd -run '^TestPatchUserSkill$' -count=1`
- `go test ./codersdk ./coderd/database/db2sdk`
- `make lint`
- pre-commit hook on `97fd58108d`

coderd/coderd.go

@@ -1184,6 +1184,19 @@ func New(options *Options) *API {
 				})
 			})
 		})
+		r.Route("/users/{user}/skills", func(r chi.Router) {
+			r.Use(
+				apiKeyMiddleware,
+				httpmw.ExtractUserParam(options.Database),
+			)
+			r.Post("/", api.postUserSkill)
+			r.Get("/", api.getUserSkills)
+			r.Route("/{skillName}", func(r chi.Router) {
+				r.Get("/", api.getUserSkill)
+				r.Patch("/", api.patchUserSkill)
+				r.Delete("/", api.deleteUserSkill)
+			})
+		})
 		r.Route("/chats", func(r chi.Router) {
 			r.Use(
 				apiKeyMiddleware,