Fix low severity security findings

[huangzesen]

Fixes #6

Summary

• stop returning absolute node paths in node_info and manager list/spawn responses
• remove OS PID from heartbeat payloads
• replace Python hash() covenant fingerprints with stable SHA-256
• confine LICC inbox writes to the configured project root and validate MCP names as single path segments
• add size limits for mail bodies and JSON attachments

Tests

• PYTHONPATH=src pytest -q