feat: improve state migration and delivery visibility

[davidahmann]

Summary

Improves Mill's everyday delivery path and local state safety without changing its authority model.

• Makes start, status, and redacted stats easier to discover; documents the CLI/schema public interface, a glossary, delivery-access guidance, and archives the historical greenfield plan.
• Replaces implicit SQLite state updates with an atomic numbered migration ledger. Supported v1/v2 records migrate to v3; future, incomplete, conflicting, and tampered migration state fails closed.
• Adds aggregate state statistics with no task, prompt, path, credential, raw output, event-payload, or delivery-receipt exposure.
• Adds a deliberately narrow worker-adapter registry while retaining codex-cli as the only supported adapter.
• Makes local validation scripts and test fixtures honor MILL_GIT_PATH, so a non-system Git installation can run the full suite without changing production runtime environment.

Validation

• npm run format:check
• npm run lint
• npm run typecheck
• npm run docs:check
• npm run check:workflows
• npm run schemas:check
• 356 tests pass with coverage thresholds: 89.05% lines, 88.48% statements, 95.24% functions, 82.00% branches.
• npm run test:package
• node dist/cli.js --json --cwd . audit

The exact-candidate audit passed all product, code, UX, accessibility, security, dependency, architecture, operations, and release checks.