fix: recover empty goal state files

[danyel117]

Summary

• treat empty, whitespace-only, BOM-only, and boundary NUL-padded state files as empty state
• emit a one-time warning per recovered state path while preserving malformed non-empty files
• keep async and sync readers aligned and let the next mutation persist canonical state atomically
• cover V1 prompt hooks, V2 tools, real mutations, BOM input, warnings, and corrupt-state preservation

Validation

• bun run lint
• bun run typecheck
• bun test (190 pass)
• bun run build
• bun run pack:dry-run
• two Claude Opus review rounds; final result: no merge blocker

Closes #1