feat: add personal skills settings UI and docs (#25066)

> Mux updated this PR on behalf of Mike.

## Summary

- Add experimental personal skills API helpers and an Agents settings UI
for listing, creating, editing, deleting, and importing SKILL.md
content.
- Add docs, Storybook coverage, and unit tests for backend-compatible
SKILL.md parsing.
- Address review feedback by simplifying frontmatter scalar parsing,
clarifying the UI parser scope, defaulting personal skill queries to
`me`, and patching React Query caches after create, update, and delete.
- Merge latest `main` and resolve the Agents sidebar refactor conflicts.

## Validation

- pre-commit hook
- `go test ./codersdk/workspacesdk -run TestParseSkillFrontmatter
-count=1`
- `go test ./coderd/x/chatd/chattool -run 'Test' -count=1`
- `cd site && pnpm test --
src/pages/AgentsPage/utils/personalSkills.test.ts
src/api/queries/userSkills.test.ts src/utils/fileSize.test.ts
--runInBand`
- `cd site && pnpm lint:types`
- `cd site && pnpm lint:check`

coderd/x/chatd/chattool/skill.go

@@ -266,9 +266,8 @@ const DefaultSkillMetaFile = "SKILL.md"
 // ReadSkillOptions configures the read_skill and read_skill_file
 // tools.
 type ReadSkillOptions struct {
-	GetWorkspaceConn func(context.Context) (workspacesdk.AgentConn, error)
-	GetSkills        func() []SkillMeta
-
+	GetWorkspaceConn      func(context.Context) (workspacesdk.AgentConn, error)
+	GetSkills             func() []SkillMeta
 	ResolveAlias          func(string) (skillspkg.ResolvedSkill, error)
 	LoadPersonalSkillBody func(context.Context, string) (skillspkg.ParsedSkill, error)
 }