refactor(startup): inject coordinator dependencies

[pablontiv]

Summary

Related to #48, phase 1c.3b.

• removes the five mutable startup coordination globals: startupMutationWait, startupTryAcquire, startupAcquire, startupPrepareIndex, and startupSync
• introduces one concrete startup coordinator per root-command invocation with production dependencies supplied by its constructor
• updates startup tests to use coordinator-local fakes and adds parallel isolation coverage
• preserves owner, follower, remediation, context, progress, byte, error, and exactly-once release behavior

Evidence

• candidate review accepted after rebasing onto 9217ec5b5b59a29774e1e89b581ee246a3a94864
• stable patch-id unchanged and range-diff equivalent across the rebase
• just ci passed with 85.3% total coverage
• go test -race ./... passed
• pre-push hook passed