feat: define native orchestration policy

[mlaccetti]

Summary

• Make the default orchestrator keep coordination small, delegate bounded work, parallelize independent tasks, and use native lifecycle events or callbacks rather than polling.
• Specify gh-stacking for dependent pull requests and the exact @mergify queue comment after checks are green, followed by event-driven verification of Mergify, merge, and post-merge CI.
• Keep worker assignments and handoffs as ordinary conversation with no required schema, status enum, or serialized artifact.
• Configure Codex workers for Luna Medium and update harness prompts and docs to match.
• Remove the obsolete deterministic pass spec.

Verification

• golangci-lint run ./...
• ./scripts/check-no-sleep.sh
• go test ./... -race -count=1
• git diff --check