fix(backfill): normalize commit_date via datetime() in time-proximity query

SQLite datetime comparison fails lexicographically across ISO-T and
space-separator formats: '2026-03-27 18:00:14' < '2026-03-27T17:43:04+00:00'
because space (0x20) < T (0x54). PRs merged same-day but earlier than the
commit hour were silently excluded from the time-proximity cascade.

Shaga's 3 stigmergic-coordination claims resolved to PR #2032 (later, wrong)
instead of #2025 (earlier, correct). Fixed by wrapping both sides in
datetime(), which normalizes to space-separator before comparison.

Verified: all 3 Shaga claims now resolve to #2025 via git_time_proximity.
No change to totals (126 originator events, 5 proximity hits) — the fix
corrects WHICH PR each proximity-matched claim resolves to, not whether.

Caught by Ganymede review of 1d6b515.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/backfill-events.py

@@ -344,11 +344,18 @@ def find_pr_for_claim(
                 )
                 commit_date = date_out.strip() if date_out.strip() else None
                 if commit_date:
+                    # git %aI returns ISO 8601 with T-separator; prs.merged_at
+                    # uses SQLite's space-separator. Lexicographic comparison
+                    # fails across formats (space<T), so normalize commit_date
+                    # via datetime() before comparing. Without this, PRs merged
+                    # within the same calendar day but earlier than the commit
+                    # hour are silently excluded (caught by Ganymede review —
+                    # Shaga's #2025 was dropped in favor of later #2032).
                     row = conn.execute(
                         """SELECT number FROM prs
                            WHERE status='merged'
-                             AND merged_at >= ?
-                             AND merged_at <= datetime(?, '+24 hours')
+                             AND merged_at >= datetime(?)
+                             AND merged_at <= datetime(datetime(?), '+24 hours')
                              AND (branch LIKE 'leo/%' OR branch LIKE 'theseus/%'
                                   OR branch LIKE 'rio/%' OR branch LIKE 'astra/%'
                                   OR branch LIKE 'vida/%' OR branch LIKE 'clay/%')