Implement built-in TechPack for first-party components

[bguidolim]

Context

The update-check SessionStart hook is a first-party engine feature managed via ad-hoc static methods on UpdateChecker (addHook, removeHook, stripHooks, syncHook). This works but doesn't participate in the standard TechPack convergence engine — no artifact tracking, no derived doctor checks, no template support.

If mcs needs to deliver additional first-party artifacts (templates, settings, more hooks), the ad-hoc approach won't scale.

Proposed Change

Create a lightweight BuiltInPack conforming to the TechPack protocol with:

• The update-check hook as a component
• Future first-party templates (e.g., mcs usage instructions in CLAUDE.md)
• Full participation in the sync convergence engine (artifact tracking, doctor checks)

This replaces the current ad-hoc hook injection in ProjectSyncStrategy, GlobalSyncStrategy, and UpdateChecker.syncHook.

Design Notes

• Not a "bundled tech pack" in the old sense (general-purpose configurations were removed per Revisit whether pack version field is needed #144)
• A minimal, engine-owned pack for mcs's own operational features
• Components gated by MCSConfig preferences