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/database/db2sdk/db2sdk.go

@@ -2046,3 +2046,37 @@ func UserSecrets(secrets []database.ListUserSecretsRow) []codersdk.UserSecret {
 	}
 	return result
 }
+
+// UserSkill converts a database UserSkill to an SDK UserSkill.
+func UserSkill(skill database.UserSkill) codersdk.UserSkill {
+	return codersdk.UserSkill{
+		UserSkillMetadata: codersdk.UserSkillMetadata{
+			ID:          skill.ID,
+			Name:        skill.Name,
+			Description: skill.Description,
+			CreatedAt:   skill.CreatedAt,
+			UpdatedAt:   skill.UpdatedAt,
+		},
+		Content: skill.Content,
+	}
+}
+
+// UserSkillMetadata converts database user skill metadata to an SDK UserSkillMetadata.
+func UserSkillMetadata(skill database.ListUserSkillMetadataByUserIDRow) codersdk.UserSkillMetadata {
+	return codersdk.UserSkillMetadata{
+		ID:          skill.ID,
+		Name:        skill.Name,
+		Description: skill.Description,
+		CreatedAt:   skill.CreatedAt,
+		UpdatedAt:   skill.UpdatedAt,
+	}
+}
+
+// UserSkillMetadataList converts database user skill metadata rows to SDK values.
+func UserSkillMetadataList(rows []database.ListUserSkillMetadataByUserIDRow) []codersdk.UserSkillMetadata {
+	metadata := make([]codersdk.UserSkillMetadata, 0, len(rows))
+	for _, row := range rows {
+		metadata = append(metadata, UserSkillMetadata(row))
+	}
+	return metadata
+}