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/chatmodelconfigs.sql

@@ -0,0 +1,129 @@
+-- name: GetChatModelConfigByID :one
+SELECT
+    *
+FROM
+    chat_model_configs
+WHERE
+    id = @id::uuid
+    AND deleted = FALSE;
+
+-- name: GetDefaultChatModelConfig :one
+SELECT
+    *
+FROM
+    chat_model_configs
+WHERE
+    is_default = TRUE
+    AND deleted = FALSE;
+
+-- name: GetChatModelConfigByProviderAndModel :one
+SELECT
+    *
+FROM
+    chat_model_configs
+WHERE
+    provider = @provider::text
+    AND model = @model::text
+    AND deleted = FALSE
+ORDER BY
+    updated_at DESC,
+    created_at DESC,
+    id DESC
+LIMIT 1;
+
+-- name: GetChatModelConfigs :many
+SELECT
+    *
+FROM
+    chat_model_configs
+WHERE
+    deleted = FALSE
+ORDER BY
+    provider ASC,
+    model ASC,
+    updated_at DESC,
+    id DESC;
+
+-- name: GetEnabledChatModelConfigs :many
+SELECT
+    cmc.*
+FROM
+    chat_model_configs cmc
+JOIN
+    chat_providers cp ON cp.provider = cmc.provider
+WHERE
+    cmc.enabled = TRUE
+    AND cmc.deleted = FALSE
+    AND cp.enabled = TRUE
+ORDER BY
+    cmc.provider ASC,
+    cmc.model ASC,
+    cmc.updated_at DESC,
+    cmc.id DESC;
+
+-- name: InsertChatModelConfig :one
+INSERT INTO chat_model_configs (
+    provider,
+    model,
+    display_name,
+    created_by,
+    updated_by,
+    enabled,
+    is_default,
+    context_limit,
+    compression_threshold,
+    options
+) VALUES (
+    @provider::text,
+    @model::text,
+    @display_name::text,
+    sqlc.narg('created_by')::uuid,
+    sqlc.narg('updated_by')::uuid,
+    @enabled::boolean,
+    @is_default::boolean,
+    @context_limit::bigint,
+    @compression_threshold::integer,
+    @options::jsonb
+)
+RETURNING
+    *;
+
+-- name: UpdateChatModelConfig :one
+UPDATE
+    chat_model_configs
+SET
+    provider = @provider::text,
+    model = @model::text,
+    display_name = @display_name::text,
+    updated_by = sqlc.narg('updated_by')::uuid,
+    enabled = @enabled::boolean,
+    is_default = @is_default::boolean,
+    context_limit = @context_limit::bigint,
+    compression_threshold = @compression_threshold::integer,
+    options = @options::jsonb,
+    updated_at = NOW()
+WHERE
+    id = @id::uuid
+    AND deleted = FALSE
+RETURNING
+    *;
+
+-- name: UnsetDefaultChatModelConfigs :exec
+UPDATE
+    chat_model_configs
+SET
+    is_default = FALSE,
+    updated_at = NOW()
+WHERE
+    is_default = TRUE
+    AND deleted = FALSE;
+
+-- name: DeleteChatModelConfigByID :exec
+UPDATE
+    chat_model_configs
+SET
+    deleted = TRUE,
+    deleted_at = NOW(),
+    updated_at = NOW()
+WHERE
+    id = @id::uuid;