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/models.go

@@ -1294,6 +1294,7 @@ type ChatMode string
 
 const (
 	ChatModeComputerUse ChatMode = "computer_use"
+	ChatModeExplore     ChatMode = "explore"
 )
 
 func (e *ChatMode) Scan(src interface{}) error {
@@ -1333,7 +1334,8 @@ func (ns NullChatMode) Value() (driver.Value, error) {
 
 func (e ChatMode) Valid() bool {
 	switch e {
-	case ChatModeComputerUse:
+	case ChatModeComputerUse,
+		ChatModeExplore:
 		return true
 	}
 	return false
@@ -1342,6 +1344,7 @@ func (e ChatMode) Valid() bool {
 func AllChatModeValues() []ChatMode {
 	return []ChatMode{
 		ChatModeComputerUse,
+		ChatModeExplore,
 	}
 }