Sparse checkout and partial clone

[Ismail-elkorchi]

Acceptance Criteria

• Phase 15 acceptance criteria are implemented for Sparse checkout and partial clone.
• Activated invariants for phase 15 are satisfied by code and tests.

Checklist

• Branch name matches phase branch.
• Pull request title matches phase title.
• File changes conform to phase policy.
• spec/state.yaml updated to phase 15.
• Required commands pass.

Simulated Role Reviews

Product review

Findings:

• Phase 15 user-facing scope is implemented according to the phase plan.
  Changes made:
• Added and updated source, tests, docs, and gates for phase 15.
  Evidence pointers:
• \ passes on this branch.
• Phase 15 gate artifacts are generated under .

QA review

Findings:

• Automated checks validate expected phase 15 behavior.
  Changes made:
• Added deterministic tests and assertions for active invariants.
  Evidence pointers:
• GitHub Actions \ checks are green for this PR.
• GitHub Actions \ check is green for this PR.

Security review

Findings:

• Phase 15 changes preserve policy boundaries and integrity checks.
  Changes made:
• Updated gate handlers and invariant evidence paths where required.
  Evidence pointers:
• \ passes.
• \ passes.

Performance review

Findings:

• Phase 15 additions keep runtime checks within expected CI budget.
  Changes made:
• Added targeted logic and tests for phase 15 without broad regressions.
  Evidence pointers:
• CI job durations for this PR remain in normal range.
• \ completes successfully.

Maintainer review

Findings:

• Phase 15 keeps repository structure and governance consistent.
  Changes made:
• Updated docs and state transitions aligned with the phase contract.
  Evidence pointers:
• \ reflects phase 15.
• Phase policy and branch/title checks pass.