* docs: add DoD.md repo-wide Definition of Done
Adds a stable baseline completion bar for production-ready changes,
complementing AGENTS.md, GUARDRAILS.md, CONTRIBUTING.md, TESTING.md,
and ARCHITECTURE.md. Includes core DoD, GitNexus-specific requirements,
per-package validation baseline, task-specific DoD template, and guidance
for review prompts.
* docs: expand DoD with security, observability, and agent-workflow gates
Restructure DoD.md into numbered sections and add axes that were previously
implicit: security, observability/operability, reversibility, and explicit
guardrails for agent-assisted workflow (scope match, evidence-based edits,
pre-edit impact analysis, embeddings preservation).
Expand the validation baseline to reflect the real CI shape (shared-first
build ordering, prettier, setup-gitnexus action, CHANGELOG ownership) and
add a Review Gates checklist plus a "Not Done" signals section that flags
contract drift, language leakage into shared code, and unrelated churn.
