Add timeout, async, and statusMessage fields to HookEntry

[bguidolim]

Phase 1: Hook Handler Fields

Context

The Claude Code hooks documentation now defines additional fields on hook handlers that mcs doesn't model. These are small additions to the existing Settings.HookEntry struct.

Fields to Add

Field	Type	Description
timeout	Int?	Seconds before canceling. Defaults: 600 (command), 30 (prompt), 60 (agent)
async	Bool?	If true, runs in the background without blocking
statusMessage	String?	Custom spinner message displayed while the hook runs

Files to Modify

• Sources/mcs/Core/Settings.swift — add fields to HookEntry
• Sources/mcs/ExternalPack/ExternalPackManifest.swift — add corresponding YAML fields to ExternalHookEntry (if exists) or the hook shorthand
• Tests for round-tripping

Acceptance Criteria

• HookEntry supports timeout, async, and statusMessage as optional Codable fields
• Pack authors can specify these in techpack.yaml hook definitions
• Auto-derived hook entries pass through these fields when specified
• Existing settings files without these fields continue to work (backward compatible)

Reference

• Hooks reference — Common fields
• Hooks reference — Command hook fields