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>

cli/testdata/coder_server_--help.golden

@@ -62,6 +62,9 @@ OPTIONS:
           Separate multiple experiments with commas, or enter '*' to opt-in to
           all available experiments.
 
+      --external-auth-github-default-provider-enable bool, $CODER_EXTERNAL_AUTH_GITHUB_DEFAULT_PROVIDER_ENABLE (default: true)
+          Enable the default GitHub external auth provider managed by Coder.
+
       --postgres-auth password|awsiamrds, $CODER_PG_AUTH (default: password)
           Type of auth to use when connecting to postgres. For AWS RDS, using
           IAM authentication (awsiamrds) is recommended.

cli/testdata/server-config.yaml.golden

@@ -564,6 +564,9 @@ supportLinks: []
 # External Authentication providers.
 # (default: <unset>, type: struct[[]codersdk.ExternalAuthConfig])
 externalAuthProviders: []
+# Enable the default GitHub external auth provider managed by Coder.
+# (default: true, type: bool)
+externalAuthGithubDefaultProviderEnable: true
 # Hostname of HTTPS server that runs https://github.com/coder/wgtunnel. By
 # default, this will pick the best available wgtunnel server hosted by Coder. e.g.
 # "tunnel.example.com".