fix: allow identical copyPackFile source across components

[bguidolim]

Summary

PR #320 introduced intra-pack duplicate-destination validation that incorrectly rejected a legitimate pattern: one physical hook script registered against multiple hook events. Because hookEvent is singular per component, authors must declare N components — all copying the same source to the same destination — which is an identity, not a collision. This PR tightens the validator so it only throws when sources actually differ.

Changes

• ExternalPackManifest.validate() now tracks (componentID, source) per destination group. For groups with more than one entry, an allSatisfy check collapses the identity case (every entry copies the same source → benign, skipped). Real conflicts (distinct sources for the same destination) still throw ManifestError.duplicateDestination.
• Replaced the "generic" string fallback with ExternalCopyFileType.generic.rawValue to drop the stringly-typed literal.
• Removed a force-unwrap on the dictionary lookup by iterating sorted key-value pairs directly.
• Added allowSameSourceDestinationAcrossHookEvents test covering two hook components sharing both source and destination. Existing rejectDuplicate* tests still cover the real-collision branch.

Context

Real-world hit: the mcs-memory pack declares hook-sync-memories (SessionStart) and hook-reindex-memories (UserPromptSubmit), both copying hooks/sync-memories.sh → sync-memories.sh. After PR #320, mcs sync --global warned:

[WARN] Skipping pack 'memory': Invalid manifest for pack 'memory':
  Duplicate copyPackFile destination 'sync-memories.sh' (fileType: hook)
  in components: memory.hook-sync-memories, memory.hook-reindex-memories

This is the same file being registered against two hook events, not two files fighting for one destination. The validator should only catch the latter.

A more architecturally elegant alternative — letting hookEvent accept an array so one component registers for multiple events — was considered but deferred. That change ripples through ~13 files (HookRegistration.event, encoder/decoder, sync strategies, PackArtifactRecord.hookCommands, doctor checks, export/discovery, collision resolver) and deserves its own PR.

ExternalCopyPackFileConfig has exactly three fields (source, destination, fileType?) with no hidden mode/permissions/ownership, so source equality is a complete identity check.

Test plan

• swift test --filter MCSTests.ExternalPackManifestTests — 92/92 pass
• swift test — 978/978 pass
• swiftformat --lint . and swiftlint pass without violations
• End-to-end: swift run mcs pack validate ~/Developer/mcs-memory → [OK] memory — all checks passed
• Existing conflict tests (rejectDuplicateCopyPackFileDestinations, rejectDuplicateGenericFileTypeDestinations) still reject the real-collision branch

Checklist for engine changes

• Integration tests updated for new features — added unit test to ExternalPackManifestTests; integration lifecycle is unchanged (validator is the only touched path)
• Docs updated if behavior changed — N/A, this restores behavior PR feat: validate duplicate copyPackFile destinations within a single pack #320 broke