docs: full implementation plan (PRD decisions locked + specs 1-4 + impl plan)

- PRD: 20 locked decisions, open items cleared, fence fixed
- docs/TECH-SPEC-PIPELINE.md: deterministic math + 9 test vectors (deliverable #1)
- docs/ARCHITECTURE.md, DATA-MODEL.md, OPENAPI.md (deliverable #2),
  CONTENT-WOO.md, DEPLOYMENT.md (deliverable #4), IMPL-PLAN.md
- README rewritten; .gitignore added

AGENTS.md pending Ty's approval (protected file write was blocked).

.gitignore

@@ -0,0 +1,37 @@
+# Python
+__pycache__/
+*.py[cod]
+.venv/
+venv/
+.pytest_cache/
+.mypy_cache/
+.ruff_cache/
+*.egg-info/
+
+# Node
+node_modules/
+frontend/build/
+frontend/.svelte-kit/
+frontend/dist/
+
+# Env / secrets (NEVER commit)
+.env
+.env.local
+.env.*.local
+deploy/.env
+*.pem
+*.key
+
+# Data / images
+*.db
+*.sqlite3
+data/
+images/
+
+# OS / editor
+.DS_Store
+.idea/
+.vscode/
+
+# Logs
+*.log