fix(squad): separate parallel admission lanes

[sabbour]

Scope

Agentweaver-local Squad coordinator behavior only. This does not modify or propose an upstream Squad package/template release.

Summary

• separate up to five implementation streams from one sequential admission candidate
• defer final rebase, CI diagnosis, exact-head review, ledger, readiness, merge, and cleanup until promotion
• add static policy coverage for the coordinator contract

Validation

• npm run validate:layer
• node --test scripts/azure/tests/.test.mjs scripts/changesets/tests/.test.mjs scripts/ci/tests/*.test.mjs

Closes #1561