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/queries.sql.go

@@ -19954,6 +19954,18 @@ func (q *sqlQuerier) GetChatDesktopEnabled(ctx context.Context) (bool, error) {
 	return enable_desktop, err
 }
 
+const getChatExploreModelOverride = `-- name: GetChatExploreModelOverride :one
+SELECT
+	COALESCE((SELECT value FROM site_configs WHERE key = 'agents_chat_explore_model_override'), '') :: text AS model_config_id
+`
+
+func (q *sqlQuerier) GetChatExploreModelOverride(ctx context.Context) (string, error) {
+	row := q.db.QueryRowContext(ctx, getChatExploreModelOverride)
+	var model_config_id string
+	err := row.Scan(&model_config_id)
+	return model_config_id, err
+}
+
 const getChatIncludeDefaultSystemPrompt = `-- name: GetChatIncludeDefaultSystemPrompt :one
 SELECT
     COALESCE(
@@ -20311,6 +20323,16 @@ func (q *sqlQuerier) UpsertChatDesktopEnabled(ctx context.Context, enableDesktop
 	return err
 }
 
+const upsertChatExploreModelOverride = `-- name: UpsertChatExploreModelOverride :exec
+INSERT INTO site_configs (key, value) VALUES ('agents_chat_explore_model_override', $1)
+ON CONFLICT (key) DO UPDATE SET value = $1 WHERE site_configs.key = 'agents_chat_explore_model_override'
+`
+
+func (q *sqlQuerier) UpsertChatExploreModelOverride(ctx context.Context, value string) error {
+	_, err := q.db.ExecContext(ctx, upsertChatExploreModelOverride, value)
+	return err
+}
+
 const upsertChatIncludeDefaultSystemPrompt = `-- name: UpsertChatIncludeDefaultSystemPrompt :exec
 INSERT INTO site_configs (key, value)
 VALUES (