feat: add Explore mode as subagent-only modality (#24448)

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

Introduce Explore mode, a read-only subagent modality for delegated
discovery and code investigation.

## What

Adds a `spawn_explore_agent` tool that creates child chats restricted to
read-only operations. An admin can optionally configure a
deployment-wide
model override so Explore subagents use a model optimized for large
context
or reasoning without changing the root chat's model.

### Backend

- New `ChatModeExplore` enum value (migration 000471).
- `spawn_explore_agent` tool definition with read-only allowlist:
`read_file`, `execute`, `process_output`, `read_skill`,
`read_skill_file`.
  Write tools, file editors, and nested subagent spawning are blocked.
- Deployment config storage for the Explore model override
  (`agents_chat_explore_model_override` in `site_configs`).
- Model resolution hierarchy: configured override, then current turn
model,
then global default. Silent fallback with warning log when the override
  becomes unavailable.
- RBAC: `AsChatd` for daemon reads, `ActionRead` and `ActionUpdate` on
  `ResourceDeploymentConfig` for admin API calls.
- Plan mode root chats can use `spawn_explore_agent` for read-only
research,
  matching the planning prompt guidance.
- The Explore override config API now reports malformed saved overrides
as
  "treated as unset" so admins can clear them explicitly.

### Frontend

- `ExploreModelOverrideSettings` component in admin agent behavior
settings.
  Uses `ModelSelector`, handles unavailable model warnings, and supports
  explicit Save and Clear actions.
- Malformed saved overrides show a warning and require an explicit Save
to
  clear, instead of Clear auto-submitting behind the scenes.

### Tests

- Integration: `TestExploreSubagentIsReadOnly` (full spawn flow, tool
  verification, prompt overlay, DB state).
- Unit: tool allowlist tests for explore, plan, and default modes.
- Internal: model override resolution with valid, invalid UUID,
disabled, and
  unconfigured override scenarios.
- RBAC: `dbauthz_test.go` for `GetChatExploreModelOverride` and
  `UpsertChatExploreModelOverride`.
- API: admin set and clear, malformed stored override reporting,
disabled
  model rejection, non-admin denial.

coderd/database/dbauthz/dbauthz.go

@@ -833,7 +833,7 @@ func AsWorkspaceBuilder(ctx context.Context) context.Context {
 }
 
 // AsChatd returns a context with an actor scoped to the chat
-// daemon's background worker. It can manage chats and read
+// daemon's background worker. It can manage chats and access
 // workspaces and deployment config, but nothing else.
 func AsChatd(ctx context.Context) context.Context {
 	return As(ctx, subjectChatd)
@@ -2679,6 +2679,13 @@ func (q *querier) GetChatDiffStatusesByChatIDs(ctx context.Context, chatIDs []uu
 	return q.db.GetChatDiffStatusesByChatIDs(ctx, chatIDs)
 }
 
+func (q *querier) GetChatExploreModelOverride(ctx context.Context) (string, error) {
+	if err := q.authorizeContext(ctx, policy.ActionRead, rbac.ResourceDeploymentConfig); err != nil {
+		return "", err
+	}
+	return q.db.GetChatExploreModelOverride(ctx)
+}
+
 func (q *querier) GetChatFileByID(ctx context.Context, id uuid.UUID) (database.ChatFile, error) {
 	file, err := q.db.GetChatFileByID(ctx, id)
 	if err != nil {
@@ -7296,6 +7303,13 @@ func (q *querier) UpsertChatDiffStatusReference(ctx context.Context, arg databas
 	return q.db.UpsertChatDiffStatusReference(ctx, arg)
 }
 
+func (q *querier) UpsertChatExploreModelOverride(ctx context.Context, value string) error {
+	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
+		return err
+	}
+	return q.db.UpsertChatExploreModelOverride(ctx, value)
+}
+
 func (q *querier) UpsertChatIncludeDefaultSystemPrompt(ctx context.Context, includeDefaultSystemPrompt bool) error {
 	if err := q.authorizeContext(ctx, policy.ActionUpdate, rbac.ResourceDeploymentConfig); err != nil {
 		return err

coderd/database/dbauthz/dbauthz_test.go

@@ -834,6 +834,10 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().GetChatDesktopEnabled(gomock.Any()).Return(false, nil).AnyTimes()
 		check.Args().Asserts()
 	}))
+	s.Run("GetChatExploreModelOverride", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().GetChatExploreModelOverride(gomock.Any()).Return("", nil).AnyTimes()
+		check.Args().Asserts(rbac.ResourceDeploymentConfig, policy.ActionRead)
+	}))
 	s.Run("GetChatPlanModeInstructions", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
 		dbm.EXPECT().GetChatPlanModeInstructions(gomock.Any()).Return("", nil).AnyTimes()
 		check.Args().Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
@@ -1135,6 +1139,10 @@ func (s *MethodTestSuite) TestChats() {
 		dbm.EXPECT().UpsertChatDesktopEnabled(gomock.Any(), false).Return(nil).AnyTimes()
 		check.Args(false).Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
 	}))
+	s.Run("UpsertChatExploreModelOverride", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
+		dbm.EXPECT().UpsertChatExploreModelOverride(gomock.Any(), "").Return(nil).AnyTimes()
+		check.Args("").Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
+	}))
 	s.Run("UpsertChatPlanModeInstructions", s.Mocked(func(dbm *dbmock.MockStore, _ *gofakeit.Faker, check *expects) {
 		dbm.EXPECT().UpsertChatPlanModeInstructions(gomock.Any(), "").Return(nil).AnyTimes()
 		check.Args("").Asserts(rbac.ResourceDeploymentConfig, policy.ActionUpdate)
@@ -6099,9 +6107,11 @@ func TestAsChatd(t *testing.T) {
 			require.NoError(t, err, "workspace %s should be allowed", action)
 		}
 
-		// DeploymentConfig read.
+		// DeploymentConfig reads are allowed, but writes are not.
 		err := auth.Authorize(ctx, actor, policy.ActionRead, rbac.ResourceDeploymentConfig)
 		require.NoError(t, err, "deployment config read should be allowed")
+		err = auth.Authorize(ctx, actor, policy.ActionUpdate, rbac.ResourceDeploymentConfig)
+		require.Error(t, err, "deployment config update should not be allowed")
 
 		// User read_personal (needed for GetUserChatCustomPrompt).
 		err = auth.Authorize(ctx, actor, policy.ActionReadPersonal, rbac.ResourceUser)