build: derive generated-file stamps from git commit time, not wall clock

Two builds of the same source tree could publish different artifacts
because buildEmbeddedPatterns.ts and buildTypeEmbeddings.ts stamped
their generated output with new Date().toISOString(). Route both
generators' "Generated:" header and the generatedAt runtime field
through a shared resolver: newest git commit time among the
generator's inputs (script + source data), falling back to the stamp
already present in the previous output when git is unavailable (e.g.
a published tarball build), and finally to a fixed epoch value. Every
fallback logs to stderr so degradation is never silent.

Regenerated both committed output files once so the tree carries
deterministic stamps; no other content changed.

src/neural/embeddedPatterns.ts

@@ -2,7 +2,7 @@
  * 🧠 BRAINY EMBEDDED PATTERNS
  * 
  * AUTO-GENERATED - DO NOT EDIT
- * Generated: 2026-07-02T21:43:26.976Z
+ * Generated: 2025-09-29T10:10:00-07:00
  * Patterns: 220
  * Coverage: 94-98% of all queries
  *