Add person_metrics_export.py for external metric consumers

[bkraeling]

Issue

Surfacing a person's /team/<slug> card metrics (and their σ values) in an external dashboard meant scraping the HTML page — brittle, and it re-ran the expensive whole-team fetch on every view.

Solution

Adds person_metrics_export.py, a small read-only CLI that exports one person's card metrics + σ as JSON for arbitrary day windows. It calls _build_person_context, so the output matches the web page exactly — same values, same trimmed-baseline σ, same +/− orientation. No existing modules are changed and no new API surface is added.

python person_metrics_export.py --slug alice                       # JSON to stdout
python person_metrics_export.py --slug alice --windows 7,30,90 --out metrics.json

Output shape:

{
  "slug": "alice",
  "generated_at": 1756000000.0,
  "order": ["prs_merged", "..."],
  "labels": {"prs_merged": "PRs Merged"},
  "lower_is_better": ["avg_all_time_to_fix", "..."],
  "windows": {
    "7": {"prs_merged": {"value": 24, "sigma": "+0.3σ", "z": 0.3}}
  }
}

Notes:

• --slug is required; --windows defaults to 7,30,90; output goes to stdout unless --out is given.
• z is the σ label parsed to a float so consumers can threshold/colour without re-parsing (the label uses a Unicode minus).
• Schedule-variance values are normalized to a compact signed form (+1d late / -1d early / 0d on time) for narrow displays.
• The whole-team GitHub + Linear fetch is expensive, so this is intended to run on a schedule (e.g. daily), not per page view.

To Test

• python person_metrics_export.py --slug <your-slug> prints JSON; values and σ match that person's /team/<slug> page for the default window
• --windows 7,30,90 returns all three windows; --out <path> writes the file
• Someone not on the engineering team returns metrics with no σ entries (relative metrics are engineering-only)
• ruff check ., ruff format --check ., mypy ., vulture . --config pyproject.toml, and the unit tests pass

Verified locally: all four CI gates pass on the new file, and the existing 194 unit tests still pass.

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