fix: handle .stignore during initial sync

[acmore]

Summary

• shrink the remote materialization expectation when the current local tracked file count drops during two-phase sync
• prevent initial sync from waiting on stale pre-.stignore file counts after excluded files disappear from the tracked set
• add regression coverage for the file-count expectation helper

Verification

• go test ./internal/cli
• .venv/bin/pre-commit run --all-files --hook-stage manual okdev-gofmt