ci: migrate to Nix-based build + deploy

- Add flake.nix with 4 packages: web, api, mcp, worker
  Each builds via bun in CI sandbox, packages dist/.next + node_modules
  into Nix store, wraps with makeBinaryWrapper
- Rewrite ci.yml: test (bun typecheck+tests) → build-and-deploy (Nix matrix)
  nix build → nix copy → nix-env --set + systemctl restart on VPS
  No tarball+SSH, no VPS builds
- Gitignore content/**/*.md (DB is source of truth)
- Update AGENTS.md architecture docs

.gitignore

@@ -43,3 +43,9 @@ next-env.d.ts
 # monorepo / turbo
 .turbo/
 dist/
+
+# MCPedia content — DB is source of truth via MCP API (create_document/update_document)
+# Filesystem .md files are auto-generated backups, not tracked in git.
+# To edit content: use the MCP create_document/update_document tools (requires x-webhook-secret)
+content/**/*.md
+!content/**/.gitkeep