fix: disambiguate hook block reasons to name MemPalace explicitly (#666)

Replace "your memory system" with explicit MemPalace references and
tool names (mempalace_diary_write, mempalace_add_drawer, mempalace_kg_add)
in stop and precompact hook block reasons. This prevents Claude Code from
misinterpreting the hook as a native auto-memory save instruction.

Updated in both Python (hooks_cli.py) and standalone shell scripts.

Also fix CONTRIBUTING.md Getting Started to show the fork-first workflow,
matching the PR Guidelines section.

CONTRIBUTING.md

@@ -5,8 +5,11 @@ Thanks for wanting to help. MemPalace is open source and we welcome contribution
 ## Getting Started
 
 ```bash
-git clone https://github.com/milla-jovovich/mempalace.git
+# Fork the repo on GitHub first, then clone your fork
+git clone https://github.com/<your-username>/mempalace.git
 cd mempalace
+git remote add upstream https://github.com/milla-jovovich/mempalace.git
+
 pip install -e ".[dev]"    # installs with dev dependencies (pytest, build, twine)
 ```