[codex] Close tail client on repair-loop exits

[steipete]

Summary

• close repair-loop tail clients on every RunTail exit path
• keep the existing cancellation close path to unblock Tail, guarded with sync.Once
• fixes the race where Tail could return through errCh after cancellation and skip Close

Validation

• GOWORK=off go test -count=20 -race ./internal/syncer -run TestRunTailWithRepairLoopJoinsTailOnCancellation
• GOWORK=off go test -count=1 -race ./internal/syncer
• GOWORK=off go test -count=1 ./...

Context: main CI failed after the actions/cache dependency merge on the existing race-detector test TestRunTailWithRepairLoopJoinsTailOnCancellation.

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: [codex] Close tail client on repair-loop exits This is item 1/1 in the current shard. Shard 0/1.

This placeholder means the worker is alive and reading the current context. I will edit this same comment with the actual review when the claws are done clicking.

Crustacean status: shell secured, claws on keyboard, evidence pebbles being sorted.