feat: seed co-change graph from git history (cold-start fix)

The co-change differentiator was silent for every new customer until
agents produced lock activity. indexCodebase now mines co-commit pairs
from the last 300 commits (recency-weighted, bulk/solo commits skipped,
capped at 500 pairs) and uploads them to /api/v1/cochange/seed after
each index run — best-effort, never fails indexing. Migration 0012 adds
git_cochange and rebuilds cochange_neighbors to blend git seeds (x0.3)
under live lock evidence, tagging each row live/git/both. Tested
against a real throwaway git repo.