feat: port hook scripts from Python to Rust (flowctl hook auto-memory/ralph-guard)

- auto-memory: reads transcript, gemini AI summarization with regex fallback,
  saves to .flow/memory/ via flowctl memory add
- ralph-guard: enforces Ralph workflow rules (command validation, file lock
  checks, receipt verification, state tracking)
- hooks.json: auto-memory now calls bin/flowctl instead of python3
- Hook startup overhead: ~100ms (Python) → ~3ms (Rust)

1160 lines of Rust replacing 1039 lines of Python.
Zero clippy warnings. Same stdin/stdout/stderr protocol.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: z23cc