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/rbac/policy/policy.go

@@ -379,6 +379,14 @@ var RBACPermissions = map[string]PermissionDefinition{
 			ActionDelete: "delete a user secret",
 		},
 	},
+	"user_skill": {
+		Actions: map[Action]ActionDefinition{
+			ActionCreate: "create a user skill",
+			ActionRead:   "read user skill metadata and content",
+			ActionUpdate: "update user skill metadata and content",
+			ActionDelete: "delete a user skill",
+		},
+	},
 	"usage_event": {
 		Actions: map[Action]ActionDefinition{
 			ActionCreate: "create a usage event",