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/migrations/testdata/fixtures/000422_chat_provider_model_configs.up.sql

@@ -0,0 +1,114 @@
+INSERT INTO chat_providers (
+    id,
+    provider,
+    display_name,
+    api_key,
+    api_key_key_id,
+    enabled,
+    created_at,
+    updated_at
+) VALUES (
+    '0a8b2f84-b5a8-4c44-8c9f-e58c44a534a7',
+    'openai',
+    'OpenAI',
+    '',
+    NULL,
+    TRUE,
+    '2024-01-01 00:00:00+00',
+    '2024-01-01 00:00:00+00'
+);
+
+INSERT INTO chat_model_configs (
+    id,
+    provider,
+    model,
+    display_name,
+    enabled,
+    context_limit,
+    compression_threshold,
+    created_at,
+    updated_at
+) VALUES (
+    '9af5f8d5-6a57-4505-8a69-3d6c787b95fd',
+    'openai',
+    'gpt-5.2',
+    'GPT 5.2',
+    TRUE,
+    200000,
+    70,
+    '2024-01-01 00:00:00+00',
+    '2024-01-01 00:00:00+00'
+);
+
+INSERT INTO chats (
+    id,
+    owner_id,
+    last_model_config_id,
+    title,
+    status,
+    created_at,
+    updated_at
+)
+SELECT
+    '72c0438a-18eb-4688-ab80-e4c6a126ef96',
+    id,
+    '9af5f8d5-6a57-4505-8a69-3d6c787b95fd',
+    'Fixture Chat',
+    'completed',
+    '2024-01-01 00:00:00+00',
+    '2024-01-01 00:00:00+00'
+FROM users
+ORDER BY created_at, id
+LIMIT 1;
+
+INSERT INTO chat_messages (
+    chat_id,
+    created_at,
+    role,
+    content
+) VALUES (
+    '72c0438a-18eb-4688-ab80-e4c6a126ef96',
+    '2024-01-01 00:00:00+00',
+    'assistant',
+    '{"type":"text","text":"fixture"}'::jsonb
+);
+
+INSERT INTO chat_diff_statuses (
+    chat_id,
+    url,
+    pull_request_state,
+    changes_requested,
+    additions,
+    deletions,
+    changed_files,
+    refreshed_at,
+    stale_at,
+    created_at,
+    updated_at,
+    git_branch,
+    git_remote_origin
+) VALUES (
+    '72c0438a-18eb-4688-ab80-e4c6a126ef96',
+    'https://example.com/pr/1',
+    'open',
+    FALSE,
+    1,
+    0,
+    1,
+    '2024-01-01 00:00:00+00',
+    '2024-01-01 00:00:00+00',
+    '2024-01-01 00:00:00+00',
+    '2024-01-01 00:00:00+00',
+    'main',
+    'origin'
+);
+
+INSERT INTO chat_queued_messages (
+    chat_id,
+    content,
+    created_at
+) VALUES (
+    '72c0438a-18eb-4688-ab80-e4c6a126ef96',
+    '{"type":"text","text":"queued fixture"}'::jsonb,
+    '2024-01-01 00:00:00+00'
+);