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>

coderd/database/queries/workspaces.sql

@@ -399,7 +399,7 @@ WHERE
 		filtered_workspaces fw
 	ORDER BY
 		-- To ensure that 'favorite' workspaces show up first in the list only for their owner.
-		CASE WHEN owner_id = @requester_id AND favorite THEN 0 ELSE 1 END ASC,
+		CASE WHEN favorite AND owner_username = (SELECT users.username FROM users WHERE users.id = @requester_id) THEN 0 ELSE 1 END ASC,
 		(latest_build_completed_at IS NOT NULL AND
 			latest_build_canceled_at IS NULL AND
 			latest_build_error IS NULL AND