feat: add plan mode with restricted tool boundary (#24236)

> This PR was authored by Mux on behalf of Mike.

## Summary
- add persistent plan mode for chats and the chat-specific plan file
flow
- add structured planning tools such as `ask_user_question` and
`propose_plan`
- keep `write_file` and `edit_files` constrained to the chat-specific
plan file during plan turns
- allow shell exploration in plan mode, including subagents, via
`execute` and `process_output`
- block implementation-oriented, provider-native, MCP, dynamic, and
computer-use tools during plan turns
- update the chat UI, tests, and docs for the new planning flow

coderd/database/queries/chats.sql

@@ -402,6 +402,7 @@ INSERT INTO chats (
     last_model_config_id,
     title,
     mode,
+    plan_mode,
     status,
     mcp_server_ids,
     labels,
@@ -417,6 +418,7 @@ INSERT INTO chats (
     @last_model_config_id::uuid,
     @title::text,
     sqlc.narg('mode')::chat_mode,
+    sqlc.narg('plan_mode')::chat_plan_mode,
     @status::chat_status,
     COALESCE(@mcp_server_ids::uuid[], '{}'::uuid[]),
     COALESCE(sqlc.narg('labels')::jsonb, '{}'::jsonb),
@@ -518,6 +520,17 @@ WHERE
 RETURNING
     *;
 
+-- name: UpdateChatPlanModeByID :one
+UPDATE
+    chats
+SET
+    -- NOTE: updated_at is intentionally NOT touched here to avoid changing list ordering.
+    plan_mode = sqlc.narg('plan_mode')::chat_plan_mode
+WHERE
+    id = @id::uuid
+RETURNING
+    *;
+
 -- name: UpdateChatLastModelConfigByID :one
 UPDATE
     chats

coderd/database/queries/siteconfig.sql

@@ -159,6 +159,14 @@ SELECT
 INSERT INTO site_configs (key, value) VALUES ('agents_chat_system_prompt', $1)
 ON CONFLICT (key) DO UPDATE SET value = $1 WHERE site_configs.key = 'agents_chat_system_prompt';
 
+-- name: GetChatPlanModeInstructions :one
+SELECT
+	COALESCE((SELECT value FROM site_configs WHERE key = 'agents_chat_plan_mode_instructions'), '') :: text AS plan_mode_instructions;
+
+-- name: UpsertChatPlanModeInstructions :exec
+INSERT INTO site_configs (key, value) VALUES ('agents_chat_plan_mode_instructions', $1)
+ON CONFLICT (key) DO UPDATE SET value = $1 WHERE site_configs.key = 'agents_chat_plan_mode_instructions';
+
 -- name: GetChatDesktopEnabled :one
 SELECT
 	COALESCE((SELECT value = 'true' FROM site_configs WHERE key = 'agents_desktop_enabled'), false) :: boolean AS enable_desktop;