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/dbmock/dbmock.go

@@ -2417,6 +2417,21 @@ func (mr *MockStoreMockRecorder) GetChatModelConfigsForTelemetry(ctx any) *gomoc
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatModelConfigsForTelemetry", reflect.TypeOf((*MockStore)(nil).GetChatModelConfigsForTelemetry), ctx)
 }
 
+// GetChatPlanModeInstructions mocks base method.
+func (m *MockStore) GetChatPlanModeInstructions(ctx context.Context) (string, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetChatPlanModeInstructions", ctx)
+	ret0, _ := ret[0].(string)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetChatPlanModeInstructions indicates an expected call of GetChatPlanModeInstructions.
+func (mr *MockStoreMockRecorder) GetChatPlanModeInstructions(ctx any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatPlanModeInstructions", reflect.TypeOf((*MockStore)(nil).GetChatPlanModeInstructions), ctx)
+}
+
 // GetChatProviderByID mocks base method.
 func (m *MockStore) GetChatProviderByID(ctx context.Context, id uuid.UUID) (database.ChatProvider, error) {
 	m.ctrl.T.Helper()
@@ -8282,6 +8297,21 @@ func (mr *MockStoreMockRecorder) UpdateChatPinOrder(ctx, arg any) *gomock.Call {
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatPinOrder", reflect.TypeOf((*MockStore)(nil).UpdateChatPinOrder), ctx, arg)
 }
 
+// UpdateChatPlanModeByID mocks base method.
+func (m *MockStore) UpdateChatPlanModeByID(ctx context.Context, arg database.UpdateChatPlanModeByIDParams) (database.Chat, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "UpdateChatPlanModeByID", ctx, arg)
+	ret0, _ := ret[0].(database.Chat)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// UpdateChatPlanModeByID indicates an expected call of UpdateChatPlanModeByID.
+func (mr *MockStoreMockRecorder) UpdateChatPlanModeByID(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpdateChatPlanModeByID", reflect.TypeOf((*MockStore)(nil).UpdateChatPlanModeByID), ctx, arg)
+}
+
 // UpdateChatProvider mocks base method.
 func (m *MockStore) UpdateChatProvider(ctx context.Context, arg database.UpdateChatProviderParams) (database.ChatProvider, error) {
 	m.ctrl.T.Helper()
@@ -9741,6 +9771,20 @@ func (mr *MockStoreMockRecorder) UpsertChatIncludeDefaultSystemPrompt(ctx, inclu
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpsertChatIncludeDefaultSystemPrompt", reflect.TypeOf((*MockStore)(nil).UpsertChatIncludeDefaultSystemPrompt), ctx, includeDefaultSystemPrompt)
 }
 
+// UpsertChatPlanModeInstructions mocks base method.
+func (m *MockStore) UpsertChatPlanModeInstructions(ctx context.Context, value string) error {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "UpsertChatPlanModeInstructions", ctx, value)
+	ret0, _ := ret[0].(error)
+	return ret0
+}
+
+// UpsertChatPlanModeInstructions indicates an expected call of UpsertChatPlanModeInstructions.
+func (mr *MockStoreMockRecorder) UpsertChatPlanModeInstructions(ctx, value any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "UpsertChatPlanModeInstructions", reflect.TypeOf((*MockStore)(nil).UpsertChatPlanModeInstructions), ctx, value)
+}
+
 // UpsertChatRetentionDays mocks base method.
 func (m *MockStore) UpsertChatRetentionDays(ctx context.Context, retentionDays int32) error {
 	m.ctrl.T.Helper()