feat: add cross-run pass-rate trend gating

[brandonwise]

Summary

Adds a GateHistoryAnalyzer flow to persist per-run gate summaries and detect sustained pass-rate drift over time.

This closes the blind spot where each run can pass point-in-time baseline checks while quality declines gradually across many runs.

Why now

• AI agent teams are asking for runtime drift/eval-over-time workflows (Reddit + Product Hunt + GitHub signals).
• Existing --baseline checks only compare one run vs one baseline.
• This enables CI to fail on directional decline, not just single-step drops.

What changed

• Added src/agent_release_gate/history.py:
  • record_history() to append run summaries as JSON.
  • analyze_history() to compute OLS slope over recent pass rates.
  • Direction labeling: improving | stable | declining.
  • Machine-readable trend_to_dict() output.
• Extended CLI:
  • evaluate --record-history <dir> records each run summary.
  • New trend subcommand:
    • --history <dir>
    • --window <n>
    • --output <file>
    • --fail-on-regression (exit 1 on declining trend)
• Added/expanded tests:
  • tests/test_history.py for record + trend behavior.
  • tests/test_cli.py for parser coverage and CLI behavior (record-history, regression fail exit).
• Updated README with trend recording/analyzing examples.

Validation

1) Repo-level test suite

• python3 -m pytest
• ✅ 17 passed in 0.04s

2) Targeted tests for changed modules

• python3 -m pytest tests/test_history.py tests/test_cli.py
• ✅ 8 passed in 0.02s

3) Lint/type/build

• python3 -m ruff check .
• ✅ All checks passed!
• python3 -m mypy src
• ✅ Success: no issues found in 5 source files
• python3 -m build
• ✅ Successfully built agent_release_gate-0.1.0.tar.gz and agent_release_gate-0.1.0-py3-none-any.whl

4) Smoke/integration checks

• Command:
  • PYTHONPATH=src python3 -m agent_release_gate.cli trend --history "$tmpdir/history" --fail-on-regression
• Fixture: three historical runs with pass rates 0.92 -> 0.88 -> 0.84
• ✅ Exit code 1 (expected)
• ✅ Output includes:
  • "pass_rate_slope": -0.04
  • "pass_rate_direction": "declining"
  • "any_regression": true

Docs tone check (humanizer)

• node /Users/bwise/.openclaw/workspace/projects/humanizer/src/cli.js score --file README.md --ignore-code
• ✅ 🟢 0/100

Closes #4.