feat: add global chat workspace TTL setting (#23265)

- Add `agents_workspace_ttl` site config (default: whatever the template
says a.k.a. `0s`)
- Expose via GET/PUT `/api/experimental/chats/config/workspace-ttl`
- Chat tool reads setting and passes `TTLMillis` on workspace creation
- Existing autostop infrastructure handles the rest (zero changes to
LifecycleExecutor, CalculateAutostop, or activity bumping)
- ⚠️ Template-level `UserAutostopEnabled=false` overrides this global
default. Not touching this.
- Frontend: "Workspace Lifetime" control in /agents/settings Behavior
tab (admin-only)

> This PR was created with the help of Coder Agents, and has been
reviewed by several humans and robots. 🤖🤝🧑‍💻

coderd/database/querier.go

@@ -255,6 +255,9 @@ type sqlcQuerier interface {
 	GetChatUsageLimitConfig(ctx context.Context) (ChatUsageLimitConfig, error)
 	GetChatUsageLimitGroupOverride(ctx context.Context, groupID uuid.UUID) (GetChatUsageLimitGroupOverrideRow, error)
 	GetChatUsageLimitUserOverride(ctx context.Context, userID uuid.UUID) (GetChatUsageLimitUserOverrideRow, error)
+	// Returns the global TTL for chat workspaces as a Go duration string.
+	// Returns "0s" (disabled) when no value has been configured.
+	GetChatWorkspaceTTL(ctx context.Context) (string, error)
 	GetChats(ctx context.Context, arg GetChatsParams) ([]Chat, error)
 	GetConnectionLogsOffset(ctx context.Context, arg GetConnectionLogsOffsetParams) ([]GetConnectionLogsOffsetRow, error)
 	GetCryptoKeyByFeatureAndSequence(ctx context.Context, arg GetCryptoKeyByFeatureAndSequenceParams) (CryptoKey, error)
@@ -923,6 +926,7 @@ type sqlcQuerier interface {
 	UpsertChatUsageLimitConfig(ctx context.Context, arg UpsertChatUsageLimitConfigParams) (ChatUsageLimitConfig, error)
 	UpsertChatUsageLimitGroupOverride(ctx context.Context, arg UpsertChatUsageLimitGroupOverrideParams) (UpsertChatUsageLimitGroupOverrideRow, error)
 	UpsertChatUsageLimitUserOverride(ctx context.Context, arg UpsertChatUsageLimitUserOverrideParams) (UpsertChatUsageLimitUserOverrideRow, error)
+	UpsertChatWorkspaceTTL(ctx context.Context, workspaceTtl string) error
 	UpsertConnectionLog(ctx context.Context, arg UpsertConnectionLogParams) (ConnectionLog, error)
 	// The default proxy is implied and not actually stored in the database.
 	// So we need to store it's configuration here for display purposes.