fix: improve agent detection heuristics

[monperrus]

working with @tdegueul at Bellairs.

Found and fixed this bug.

# BEFORE
$ python script/detect-agents.py data/steipete-2025-12.json
Commits: 2908  |  with file cache: 1802  |  agents loaded: 44

Agent Detection — steipete-2025-12
=======================================================
  Commits with ≥1 agent signal : 65 / 2908  (2%)
  Commits with >1 agent signal : 6

Agents detected  (n=2908 commits)
─────────────────────────────────
  codex                     █████████████████████████████████████ 2%         54
  amp                       ███████ 0%                                       13
  claude_code               ██ 0%                                             3
  gemini                    █ 0%                                              1

# AFTER
$ python script/detect-agents.py data/steipete-2025-12.json
Commits: 2908  |  cached: 2808  |  full message: 0  |  agents: 44
  tip: run analyze-commit-quality.py first to fetch full messages

Agent Detection — steipete-2025-12
=======================================================
  Commits with ≥1 agent signal : 102 / 2908  (4%)
  Commits with >1 agent signal : 6

Agents detected  (n=2908 commits)
─────────────────────────────────
  codex                     █████████████████████████████████████ 3%         78
  amp                       ██████████ 1%                                    26
  claude_code               ██ 0%                                             3
  gemini                    █ 0%                                              1

really unclear why it's really hard to identify the agents used in @steipete commits. He probably prompts to remove the default commits/branches idiosyncrasies.

[rrobbes]

Thanks! I think he might be using the pi agent, which does not have obvious commit level markers.