feat(coding-agent): add startup.quiet setting to silence startup output (#777)

* feat(coding-agent): add startup.quiet setting to silence startup output

* feat(coding-agent): add startup.quiet setting to silence startup output

Adds a new setting `startup.quiet` that when set to `true` hides:
- Version and keybinding hints header
- Loaded context/skills/templates/extensions discovery info
- Model scope line

Changelog notifications are still shown so users know about updates.

Usage in ~/.pi/agent/settings.json:
{
  "startup": {
    "quiet": true
  }
}

* refactor: flatten startup.quiet to quietStartup on Settings

packages/coding-agent/src/main.ts

@@ -503,7 +503,7 @@ export async function main(args: string[]) {
 	if (mode === "rpc") {
 		await runRpcMode(session);
 	} else if (isInteractive) {
-		if (scopedModels.length > 0) {
+		if (scopedModels.length > 0 && !settingsManager.getQuietStartup()) {
 			const modelList = scopedModels
 				.map((sm) => {
 					const thinkingStr = sm.thinkingLevel ? `:${sm.thinkingLevel}` : "";