fix(#37,#38-#43,#42): narrow LLM write guard, expanduser paths, Copilot absolute paths

[FZ2000]

Fixes #37, relates to #38, #39, #40, #41, #42, #43

#37 — LLM memory write MEMORY_ALLOWED_BASE too broad

The base guard now runs at the START of apply_memory_via_llm() (before any LLM call). If a subclass sets MEMORY_SCHEMA but forgets MEMORY_ALLOWED_BASE, it gets a RuntimeError immediately rather than silently defaulting to the entire home directory. This prevents accidental whole-home write access from new appliers that copy the base pattern without setting the guard.

#38-#43 — Tool memory files not written after sync

Root cause: LLM-generated paths using tilde notation (e.g. /.claude/CLAUDE.md) were not being expanded before the security check. Path('/.claude/CLAUDE.md').resolve() on macOS resolves relative to CWD, not the user home, causing the guard to reject valid paths. Fixed by adding Path.expanduser() before .resolve() in the path validation logic.

#42 — GitHub Copilot applier uses relative CWD paths

Module-level COPILOT_INSTRUCTIONS and VSCODE_MCP_JSON constants were relative Paths. Now replaced with accessor functions (_copilot_instructions(), _vscode_mcp_json(), _copilot_instructions_dir()) evaluated at call-time using Path.cwd().resolve(), ensuring stable absolute paths.

Tests

4 new tests in test_security_llm_memory_path.py:

• RuntimeError when MEMORY_ALLOWED_BASE is None
• No error when MEMORY_ALLOWED_BASE is properly set
• Path outside allowed base is rejected with a warning
• Tilde paths inside allowed base are accepted