fix(coding-agent): preserve compaction context and check committed completion

[MelodyVAR]

Compaction can turn a blank or thinking-only model response into a nonempty summary by adding bookkeeping text, or drop the previous summary on a repeated split-turn pass with no new history. Reject empty model summaries before adding that text and retain the existing summary when no new history is available.

Add an opt-in --completion-check git-committed for print/JSON mode. It checks for a new commit, a nonempty committed diff, a clean worktree, and final answer text, then permits at most 1–3 additional prompts in the existing session. Git probes are bounded and read-only. Terminal denial, assistant errors/aborts, and session replacement suppress automatic feedback; default behavior remains unchanged. Denial recovery is covered separately by #188.

Validation:

• npm run check passed.
• pnpm run build:offline and the required ./test.sh passed with isolated credentials.
• Focused compaction and completion tests passed (171 tests across the targeted suites).

This change adds no grading inputs, automatic commits, or independent correctness judgment. The separate completion-review experiment is not included.

AI-assisted implementation and description; behavior and validation were reviewed against the source.