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/apidoc/docs.go

@@ -12783,6 +12783,11 @@ const docTemplate = `{
                 "boundary_usage:delete",
                 "boundary_usage:read",
                 "boundary_usage:update",
+                "chat:*",
+                "chat:create",
+                "chat:delete",
+                "chat:read",
+                "chat:update",
                 "coder:all",
                 "coder:apikeys.manage_self",
                 "coder:application_connect",
@@ -12987,6 +12992,11 @@ const docTemplate = `{
                 "APIKeyScopeBoundaryUsageDelete",
                 "APIKeyScopeBoundaryUsageRead",
                 "APIKeyScopeBoundaryUsageUpdate",
+                "APIKeyScopeChatAll",
+                "APIKeyScopeChatCreate",
+                "APIKeyScopeChatDelete",
+                "APIKeyScopeChatRead",
+                "APIKeyScopeChatUpdate",
                 "APIKeyScopeCoderAll",
                 "APIKeyScopeCoderApikeysManageSelf",
                 "APIKeyScopeCoderApplicationConnect",
@@ -14848,6 +14858,9 @@ const docTemplate = `{
                 "external_auth": {
                     "$ref": "#/definitions/serpent.Struct-array_codersdk_ExternalAuthConfig"
                 },
+                "external_auth_github_default_provider_enable": {
+                    "type": "boolean"
+                },
                 "external_token_encryption_keys": {
                     "type": "array",
                     "items": {
@@ -15132,9 +15145,11 @@ const docTemplate = `{
                 "workspace-usage",
                 "web-push",
                 "oauth2",
+                "agents",
                 "mcp-server-http"
             ],
             "x-enum-comments": {
+                "ExperimentAgents": "Enables agent-powered chat functionality.",
                 "ExperimentAutoFillParameters": "This should not be taken out of experiments until we have redesigned the feature.",
                 "ExperimentExample": "This isn't used for anything.",
                 "ExperimentMCPServerHTTP": "Enables the MCP HTTP server functionality.",
@@ -15150,6 +15165,7 @@ const docTemplate = `{
                 "Enables the new workspace usage tracking.",
                 "Enables web push notifications through the browser.",
                 "Enables OAuth2 provider functionality.",
+                "Enables agent-powered chat functionality.",
                 "Enables the MCP HTTP server functionality."
             ],
             "x-enum-varnames": [
@@ -15159,6 +15175,7 @@ const docTemplate = `{
                 "ExperimentWorkspaceUsage",
                 "ExperimentWebPush",
                 "ExperimentOAuth2",
+                "ExperimentAgents",
                 "ExperimentMCPServerHTTP"
             ]
         },
@@ -18099,6 +18116,7 @@ const docTemplate = `{
                 "assign_role",
                 "audit_log",
                 "boundary_usage",
+                "chat",
                 "connection_log",
                 "crypto_key",
                 "debug_info",
@@ -18144,6 +18162,7 @@ const docTemplate = `{
                 "ResourceAssignRole",
                 "ResourceAuditLog",
                 "ResourceBoundaryUsage",
+                "ResourceChat",
                 "ResourceConnectionLog",
                 "ResourceCryptoKey",
                 "ResourceDebugInfo",