Add hookMatcher support for pack-installed hooks

[bguidolim]

Phase 1: Matcher Support for Pack Authors

Context

The matcher field on HookGroup exists in the data model (for JSON round-tripping), but the engine always sets it to nil when auto-deriving hook entries from pack components. Pack authors cannot target specific tools (e.g., only run a linting hook after Edit|Write calls).

Problem

A pack that installs a PostToolUse hook for code formatting currently fires on every tool call. The author should be able to specify matcher: "Edit|Write" so it only runs when files change.

Proposed Solution

Add a hookMatcher: String? field to ExternalComponentDefinition (alongside existing hookEvent). When the engine auto-derives hook settings entries, use this value for the HookGroup.matcher field.

Example Pack YAML

components:
  - id: format-hook
    description: Auto-format on file changes
    hookEvent: PostToolUse
    hookMatcher: "Edit|Write"
    hook:
      source: hooks/format.sh
      destination: format.sh

Files to Modify

• Sources/mcs/ExternalPack/ExternalPackManifest.swift — add hookMatcher to component definition
• Sources/mcs/TechPack/Component.swift — add hookMatcher to ComponentDefinition
• Sources/mcs/Install/Configurator.swift or settings composition — use hookMatcher when creating HookGroup
• Validation: if hookMatcher is set, hookEvent must also be set
• Tests

Acceptance Criteria

• Pack authors can specify hookMatcher in component definitions
• Auto-derived HookGroup entries use the specified matcher
• Validation rejects hookMatcher without hookEvent
• Events that don't support matchers (UserPromptSubmit, Stop, etc.) get a validation warning

Reference

• Hooks reference — Matcher patterns