feat(notifications): add notifySubagents toggle to suppress subagent turn pings

A parent agent that spawns 8 subagents was producing 9 "Turn complete"
notifications per round — almost always noise. New `notifySubagents`
config flag (defaults to false) gates `turn-completed` and `turn-error`
from any tab with a `parentTabId`. The flag is intentionally NOT applied
to `permission-required` — a subagent's permission prompt still needs a
human tap to proceed, so suppressing it would silently hang the
subagent. `agent-spawned` is already top-level-only by construction.

Wiring:
- core/notifications/types.ts: NtfyConfig.notifySubagents: boolean
- core/notifications/config.ts: defaults to false; normalize() tolerates
  missing / wrong-typed values and falls back to false
- core/notifications/dispatcher.ts: new optional TabParentLookup option
  (getTabParentId). When notifySubagents=false AND the lookup returns a
  non-empty parent id string, turn-completed/turn-error are dropped.
  Lookup failures (no lookup configured, throws, returns undefined) fall
  back to "treat as top-level" so legitimate top-level events are never
  silently dropped when the DB is briefly unreadable.
- api/app.ts: wires getTabParentId via core's getTab(id)?.parentTabId
- frontend SettingsPanel.svelte: "Include subagent tabs" checkbox with
  an explanatory hint that permission prompts still fire

Tests (+9):
- 3 in config.test.ts: default-false, explicit-true, wrong-typed fallback
- 6 in dispatcher.test.ts: suppression of turn-completed/turn-error from
  subagents, no suppression when flag is true, permission-required not
  gated, graceful fallback when lookup is missing/throws/returns undefined

Live ntfy.sh round-trip re-verified (status: 200).

1 files changed, 11 insertions, 0 deletions

diff --git a/packages/api/src/app.ts b/packages/api/src/app.ts
index 24cef24..0dabb0d 100644
--- a/packages/api/src/app.ts
+++ b/packages/api/src/app.ts
@@ -24,6 +24,17 @@ export const notificationDispatcher = new NotificationDispatcher({
 			return null;
 		}
 	},
+	getTabParentId: (tabId) => {
+		try {
+			// `undefined` when the lookup fails (tab not found / DB unavailable)
+			// so the dispatcher falls back to "treat as top-level" rather than
+			// silently dropping notifications.
+			const row = getTab(tabId);
+			return row ? row.parentTabId : undefined;
+		} catch {
+			return undefined;
+		}
+	},
 });
 notificationDispatcher.attachToAgentManager(agentManager);
 notificationDispatcher.attachToPermissionManager(permissionManager);