feat: month-frozen lifetime stats ledger

[TimeToBuildBob]

Bob's lifetime numbers, recorded so they don't drift depending on when someone estimates them.

• data/ledger.csv: one row per (metric, month) with the exact query and computed_at. A month freezes at month_end + 3 days and ledger.py refuses to rewrite a frozen row. A changed definition becomes a new metric name.
• Public metrics (collect_public.py, daily Actions workflow, GitHub search with is:public): prs_merged_public, prs_opened_public, issues_opened_public, commits_public_default_branch.
• Private aggregates (collect_private.py): brain_commits and approximate sessions. Bob's host computes them and sends only integer totals via repository_dispatch (brain-side bob-stats-sync.timer, not enabled yet). They are shown separately and never summed with the public metrics.
• render.py generates LIFETIME.md, data/lifetime.json, the README table and the charts.
• Backfilled 2024-11 to 2026-09. As of 2026-09-14: 4,038 merged public PRs, 8,223 public commits, 89,128 brain commits.
• 14 tests (tests/test_ledger.py) cover the freeze rules; they pass locally.

Related: https://github.com/gptme/stats (the gptme project's stats, kept separate) and https://timetobuildbob.github.io/timeline/.

Built by a session that hit its usage limit before opening this PR, so it has had one pass of review, not two. After merge: dispatch collect.yml once, then enable bob-stats-sync.timer on Bob's host.