feat: batch progress + ETA lines during --execute runs

[seonghobae]

Summary

During --execute runs over many files, the CLI printed per-file result lines but gave no sense of overall batch progress. This PR adds a progress line after each completed file:

PROGRESS	<done>/<total>	<pct>%	ETA <mm:ss>

• ETA is extrapolated from the average per-file wall time so far x remaining files, measured with time.monotonic (immune to wall-clock jumps).
• Output style matches the existing tab-separated CONVERTED/DRY-RUN lines and is printed with flush=True so it streams live.
• Deliberately minimal: no progress bars, no new dependencies, no extra threads.
• Dry runs are unaffected — PROGRESS only appears on the --execute path.

Changes

• media_shrinker.py: new _format_progress() helper; _execute_conversions() tracks total/completed/start time and prints a PROGRESS line after each future completes. Docstrings on everything new/touched.
• tests/test_media_shrinker.py: tests for PROGRESS formatting (counts, percent, mm:ss, minutes > 1, negative-ETA clamping), a deterministic workers=1 three-file batch with a patched monotonic clock asserting exact PROGRESS lines, and a dry-run test asserting no PROGRESS output.

Gates

• coverage report --include=media_shrinker.py: 100% (581 stmts, 0 missed)
• interrogate -c pyproject.toml media_shrinker.py: PASSED (100.0%)
• unittest discover -s tests: 117 tests, no new failures (baseline 5 macOS os.listxattr errors only)

🤖 Generated with Claude Code

[opencode-agent]

OpenCode Review Overview

• Head SHA: 97c98b53f51d6fdd9be91bb1d275e0d4d1c97aa1
• Workflow run: 28760880038
• Workflow attempt: 1
• Gate result: APPROVE (exit 0)

Changed-File Evidence Map

flowchart LR
  PR["PR changed files"] --> Evidence["OpenCode bounded evidence"]
  Evidence --> S1["Changed file: media_shrinker.py"]
  S1 --> I1["repository behavior"]
  I1 --> R1["Review risk: Changed file: media_shrinker.py"]
  R1 --> V1["required checks"]
  Evidence --> S2["Test: test_media_shrinker.py"]
  S2 --> I2["regression suite"]
  I2 --> R2["Review risk: Test: test_media_shrinker.py"]
  R2 --> V2["targeted test run"]

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