Merge batch: shared preparation, exact admission

Read ../_shared/base-green.md, ../_shared/validation-gate.md and ../_shared/merge-gates.md. Their current-candidate, authority, protected-surface and credit rules govern this workflow. Historical throughput, administrative merge and blanket rebaseline exceptions are superseded by that protocol.

Batching shares discovery and conflict diagnosis. It does not make a combined board's PASS evidence valid for different intermediate squash candidates.

1. Intake and authority

Resolve each selected PR's open/draft state, current head/base, target, contributor and write authority. Read the diff and current freeze state, and check active worktree ownership. Hold any item another session is working. A request to analyze or prepare a list is not authority to merge or close it.