fix: guard mount writebacks from remote drift

[khaliqgant]

Summary

• chunk mount bulk writeback batches by serialized request size before calling /fs/bulk
• require explicit dirty/delete-pending state for tracked local writebacks instead of treating hash drift as local intent
• let authoritative remote pulls overwrite unmarked local drift and add regression coverage for drift, deletes, chunking, and pending status

Closes #182

Tests

• go test ./cmd/relayfile-cli ./cmd/relayfile-mount ./internal/mountsync/... -count=1 -timeout 240s

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: d3396503-b429-4335-bf51-359f87428326

📥 Commits

Reviewing files that changed from the base of the PR and between a9b7153 and 96ab0ab.

📒 Files selected for processing (3)

• internal/mountsync/mount_root_clobber_test.go
• internal/mountsync/syncer.go
• internal/mountsync/syncer_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/issue-182-writeback-guard-main

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[github-actions]

Relayfile Eval Review

Run: .relayfile/evals/runs/2026-05-21T12-37-59-480Z-HEAD-provider
Mode: provider
Git SHA: 7a86eb3

Passed: 4 | Needs human: 0 | Reviewable: 0 | Missing output: 0 | Failed: 0 | Skipped: 0

Human Review Cases

No reviewable human-review cases captured Relayfile output.

[devin-ai-integration]

Devin Review found 1 potential issue.

View 6 additional findings in Devin Review.