chore: adopt governance seed

[Calvinxc1]

Summary

• replace AGENTS.md with the canon coding-agent governance entrypoint
• add the full .governance seed projected from the coding-agent canon branch
• add repo-owned additive local governance for AI identity, PydanticAI output boundaries, and uv tooling

Validation

• parsed every .governance/**/*.yaml file with uv run python
• ran git diff --check for AGENTS.md and .governance

Notes

• Semantic Versioning upward signal was filed separately as ai-projects/ai-governance#4 on the lab Gitea
• .governance/overrides/policy-overrides.yaml is intentionally empty because this adoption records no true canon exceptions