fix(dual-solve): return changed file metadata

[plind-junior]

adds explicit changed-file metadata to dual-solve results so clients do not need to parse unified diffs.

changes:

• include changed_files in vouch dual-solve --json candidate payloads
• include changed_files in review-ui job/candidate payloads and choose responses
• render changed files in the browser dual-solve UI
• keep the sandbox image default as vouch/coder:latest

verified:

• .venv/bin/python -m pytest tests/test_dual_solve.py tests/test_web_dual_solve.py -q
• .venv/bin/python -m ruff check src/vouch/dual_solve.py src/vouch/cli.py src/vouch/web/dual_solve_api.py tests/test_dual_solve.py tests/test_web_dual_solve.py
• .venv/bin/python -m mypy src
• CLI smoke: vouch dual-solve ... --sandbox --dry-run --json returns changed_files: ["README.md"] for both candidates

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