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/userskills_test.go

@@ -0,0 +1,43 @@
+package coderd_test
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/coder/coder/v2/coderd/coderdtest"
+	"github.com/coder/coder/v2/coderd/rbac"
+	"github.com/coder/coder/v2/codersdk"
+	"github.com/coder/coder/v2/testutil"
+)
+
+func TestPatchUserSkill(t *testing.T) {
+	t.Parallel()
+
+	ownerRawClient := coderdtest.New(t, nil)
+	firstUser := coderdtest.CreateFirstUser(t, ownerRawClient)
+	memberRawClient, member := coderdtest.CreateAnotherUser(t, ownerRawClient, firstUser.OrganizationID)
+	memberClient := codersdk.NewExperimentalClient(memberRawClient)
+	auditorRawClient, _ := coderdtest.CreateAnotherUser(t, ownerRawClient, firstUser.OrganizationID, rbac.RoleAuditor())
+	auditorClient := codersdk.NewExperimentalClient(auditorRawClient)
+	ctx := testutil.Context(t, testutil.WaitMedium)
+
+	_, err := memberClient.CreateUserSkill(ctx, codersdk.Me, codersdk.CreateUserSkillRequest{
+		Content: userSkillTestContent("forbidden-skill", "Original body."),
+	})
+	require.NoError(t, err)
+
+	_, err = auditorClient.UpdateUserSkill(ctx, member.ID.String(), "forbidden-skill", codersdk.UpdateUserSkillRequest{
+		Content: userSkillTestContent("forbidden-skill", "Updated body."),
+	})
+	require.Error(t, err)
+	var sdkErr *codersdk.Error
+	require.ErrorAs(t, err, &sdkErr)
+	assert.Equal(t, http.StatusForbidden, sdkErr.StatusCode())
+}
+
+func userSkillTestContent(name string, body string) string {
+	return "---\nname: " + name + "\ndescription: Test skill\n---\n" + body + "\n"
+}