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/migrations/000469_chat_turn_mode.down.sql

@@ -0,0 +1,2 @@
+ALTER TABLE chats DROP COLUMN plan_mode;
+DROP TYPE chat_plan_mode;

coderd/database/migrations/000469_chat_turn_mode.up.sql

@@ -0,0 +1,2 @@
+CREATE TYPE chat_plan_mode AS ENUM ('plan');
+ALTER TABLE chats ADD COLUMN plan_mode chat_plan_mode;