test(hooks): add test coverage for directory-readme-injector hook

[Hungdoan565]

Add hook.test.ts for the directory-readme-injector hook testing the hook factory wrapper (distinct from the existing injector.test.ts which tests the injector function directly).

New tests cover:

• Hook factory returns correct handler keys
• tool.execute.before delegates to injector
• event handler for session.deleted cleanup
• Disabled hook skips processing

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Summary by cubic

Add src/hooks/directory-readme-injector/hook.test.ts to test the directory README injector hook factory. Covers Read delegation (case-insensitive), ignores other tools, clears cache on session.deleted/compacted, no-ops for unrelated events or missing sessionID; also syncs bun.lock platform binary versions to 4.5.1.

^{Written for commit 9974e5f. Summary will update on new commits. Review in cubic}