fix: address unresolved review comments on apm pack/unpack

[Copilot]

Addresses all unresolved review comments from the apm pack/apm unpack PR.

Security

Path traversal protection (packer.py, unpacker.py)

• Validate every deployed_files entry before reading/writing: reject .. components, absolute paths, and any path that resolves outside project_root / output_dir via Path.is_relative_to()
• Malicious or corrupted lockfiles with entries like .github/../../../etc/passwd are now rejected with a clear ValueError

Bug fixes

unpacker.py

• tarfile.extractall(filter="data") is Python 3.12+ only; gated behind sys.version_info >= (3, 12) with fallback to manual member checks (already present) for 3.9–3.11
• UnpackResult.extracted_dir was returning a deleted temp directory in archive mode — now returns the original bundle_path
• Lockfile error messaging: distinguishes "not found" (missing file) from "could not be parsed" (corrupt YAML) by checking lockfile_path.exists() first

Maintenance

• Move from apm_cli.commands.pack import ... from mid-file (line 313) to the top-level import block in cli.py
• Remove unused yaml / UnpackResult imports from test_unpacker.py and test_pack_unpack_e2e.py
• Add apm pack and apm unpack reference sections to docs/cli-reference.md

Type of change

• Bug fix
• New feature
• Documentation
• Maintenance / refactor

Testing

• Tested locally
• All existing tests pass
• Added tests for new functionality (if applicable)

4 new path-traversal unit tests added (2 packer, 2 unpacker). Full suite: 1387 passed.

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