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/core/settings-manager.ts

@@ -60,6 +60,7 @@ export interface Settings {
 	retry?: RetrySettings;
 	hideThinkingBlock?: boolean;
 	shellPath?: string; // Custom shell path (e.g., for Cygwin users on Windows)
+	quietStartup?: boolean;
 	collapseChangelog?: boolean; // Show condensed changelog after update (use /changelog for full)
 	extensions?: string[]; // Array of extension file paths
 	skills?: SkillsSettings;
@@ -346,6 +347,15 @@ export class SettingsManager {
 		this.save();
 	}
 
+	getQuietStartup(): boolean {
+		return this.settings.quietStartup ?? false;
+	}
+
+	setQuietStartup(quiet: boolean): void {
+		this.globalSettings.quietStartup = quiet;
+		this.save();
+	}
+
 	getCollapseChangelog(): boolean {
 		return this.settings.collapseChangelog ?? false;
 	}