feat(openagent): capture reversible file checkpoints - #196

Summary

• capture bounded before/after file contents for OpenAgent file mutations
• attach SHA-256 evidence and Base64 payloads to durable checkpoint records
• recursively snapshot directory deletes and moves within strict 500-entry / 10 MiB limits
• block symlink, oversized, and unsupported destructive mutations when a reversible checkpoint cannot be produced
• document the remaining restore/replay and terminal snapshot work honestly

Validation

• npm run build
• npm run lint (2 pre-existing warnings, 0 errors)
• npm run test:image
• git diff --check

Rust formatting, clippy, and tests run in repository CI.