feat: add experimental agents support (#22290)

feat: add AI chat system with agent tools and chat UI

Introduce the chatd subsystem and Agents UI for AI-powered chat
within Coder workspaces.

- Add chatd package with chat loop, message compaction, prompt
  management, and LLM provider integration (OpenAI, Anthropic)
- Add agent tools: create workspace, list/read templates, read/write/
  edit files, execute commands
- Add chat API endpoints with streaming, message editing, and
  durable reconnection
- Add database schema and migrations for chats, chat messages, chat
  providers, and chat model configs
- Add RBAC policies and dbauthz enforcement for chat resources
- Add Agents UI pages with conversation timeline, queued messages
  list, diff viewer, and model configuration panel
- Add comprehensive test coverage including coderd integration tests,
  chatd unit tests, and Storybook stories
- Gate feature behind experiments flag

---------

Co-authored-by: Cian Johnston <cian@coder.com>
Co-authored-by: Danielle Maywood <danielle@themaywoods.com>
Co-authored-by: Jeremy Ruppel <jeremy@coder.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

flake.lock

@@ -76,11 +76,11 @@
     },
     "nixpkgs-unstable": {
       "locked": {
-        "lastModified": 1758035966,
-        "narHash": "sha256-qqIJ3yxPiB0ZQTT9//nFGQYn8X/PBoJbofA7hRKZnmE=",
+        "lastModified": 1771369470,
+        "narHash": "sha256-0NBlEBKkN3lufyvFegY4TYv5mCNHbi5OmBDrzihbBMQ=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "8d4ddb19d03c65a36ad8d189d001dc32ffb0306b",
+        "rev": "0182a361324364ae3f436a63005877674cf45efb",
         "type": "github"
       },
       "original": {