Retrofit: fix stale command docs, extract relay-client guide

- Replace the Commands-table Lint/Type-check TODOs with the real, verified
  commands: `npm run check` (tsc --noEmit over startos/) and `npm run prettier`.
  There is no ESLint/linter; server/ JS is untooled.
- Move the client-side relay contract (env vars, /relay/* endpoints, X-Recap-*
  headers, file map) out of AGENTS.md into docs/guides/relay-client.md with
  paths: frontmatter, lazy-loaded via a .claude/rules symlink; AGENTS.md keeps
  a one-line pointer.
- Un-ignore .claude/rules/ so the guide auto-attaches in any clone, while
  .claude/ local state (worktrees, plans) stays ignored.

.gitignore

@@ -27,5 +27,7 @@ ytdlp-cache/
 # Local dev secrets
 .env
 
-# Claude Code state (worktrees, plans, etc.)
-.claude/
+# Claude Code state (worktrees, plans, etc.) — but DO track the portable
+# rules symlinks so scoped guides lazy-load in any clone.
+.claude/*
+!.claude/rules/