fix(mcp): persistent venv cache survives marketplace updates

Problem:
- Venvs in marketplace directory got deleted on every update
- Users had to manually run setup.sh and wait for full pip install
- This caused MCP servers to fail until manually fixed

Solution:
- Store venvs in external cache (~/.cache/claude-mcp-venvs/)
- Auto-repair symlinks via SessionStart hook (instant operation)
- Only run pip install on first use or when requirements change

Architecture:
  Cache (runtime) → Marketplaces → External venv cache

  The chain of symlinks ensures all three locations work:
  1. ~/.claude/plugins/cache/.../mcp-servers/* (runtime)
  2. ~/.claude/plugins/marketplaces/.../mcp-servers/* (install)
  3. ~/.cache/claude-mcp-venvs/* (persistent venvs)

Performance:
- First install: ~2-3 min (unchanged)
- After marketplace update: 0.03 sec (was 2-3 min)

Files:
- scripts/venv-repair.sh: Fast symlink restoration for hooks
- scripts/setup-venvs.sh: Full setup with external cache
- plugins/projman/hooks/startup-check.sh: Auto-repair on session start
- .gitignore: Ignore .venv symlinks

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -31,6 +31,8 @@ venv/
 ENV/
 env/
 .venv/
+.venv
+**/.venv
 
 # PyCharm
 .idea/