phase 2: per-project declarative memory store

Port of Hermes's memory_tool.py. Two files per project:
MEMORY.md (project facts, build commands, conventions) and
PITFALLS.md (anti-patterns, things tried and failed).

Features preserved from Hermes:
- § delimiter for entry separation
- Frozen snapshot at session start (prefix-cache safe)
- Char limits, not token limits (model-independent)
- Substring matching for replace/remove operations
- Atomic writes via tempfile + rename (fs_atomic)
- File locking for writer serialization
- Injection scanning for prompt injection/exfiltration
- Drift detection before mutations
- Deduplication on load

28 tests: CRUD, dedup, drift detection, injection scanning,
char budget enforcement, ambiguous match rejection, disk
persistence, frozen snapshot immutability.

Author: Yogthos