feat(models): single-source model registry via models.yaml + sync_models.py

.opencode/models/models.yaml is now the ONE file humans edit to assign a
model to an agent. tools/sync_models.py regenerates every derived surface
— the .opencode/models/*.txt files, opencode.json's `agent` block, and
each non-tier agent's .md `model:` frontmatter — so an assignment cannot
drift across surfaces. `--check` verifies with no writes and is enforced
in CI by test_model_registry_in_sync_with_manifest.

Hardened after adversarial review:
- Deletes orphan <agent>.txt files left behind when an override is
  dropped from the manifest. The dispatcher's resolver reads
  <agent>.txt before default.txt, so a stale file would silently pin
  the old model. Tier .txt files are left to sync_tier_models.py.
- Rejects a manifest key that does not name a real agent (no matching
  .opencode/agents/<name>.md) instead of silently appending a bogus
  opencode.json entry and leaving the real agent on the default model.
- Validates the regenerated opencode.json BEFORE writing it, so a bad
  render aborts cleanly instead of corrupting the file on disk.

Tier-ladder agents (task-implementor-tier-*) remain governed separately
by tiers.yaml + sync_tier_models.py and are passed through untouched.

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

.opencode/agents/git-commit-util.md

@@ -1,4 +1,5 @@
 ---
+model: local-claude/claude-haiku-4-5
 description: >
   Git commit utility. Provides safe, standardized commit, push, and rebase
   operations with proper author attribution, conflict handling, and rollback.