fix(hooks): check venv cache path before marketplace path

Startup hooks in data-platform and pr-review were checking for venvs
at the marketplace path (~/.claude/plugins/marketplaces/.../mcp-servers/)
which gets wiped on updates. The actual venvs live in the cache directory
(~/.cache/claude-mcp-venvs/) which survives updates.

This caused false "MCP venv missing" errors even when venvs existed,
wasting hours of debugging time.

Fixed hooks now check cache path first, matching the pattern used
by run.sh scripts.

Also updated docs/CANONICAL-PATHS.md with the correct venv path pattern
to prevent future occurrences.

Lesson learned: lessons/patterns/startup-hooks-must-check-venv-cache-path-first

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

plugins/data-platform/hooks/startup-check.sh

@@ -5,13 +5,18 @@
 
 PREFIX="[data-platform]"
 
-# Check if MCP venv exists
+# Check if MCP venv exists - check cache first, then local
+CACHE_VENV="$HOME/.cache/claude-mcp-venvs/leo-claude-mktplace/data-platform/.venv/bin/python"
 PLUGIN_ROOT="${CLAUDE_PLUGIN_ROOT:-$(dirname "$(dirname "$(realpath "$0")")")}"
-# MCP servers are at marketplace root, not inside plugin
 MARKETPLACE_ROOT="$(dirname "$(dirname "$PLUGIN_ROOT")")"
-VENV_PATH="$MARKETPLACE_ROOT/mcp-servers/data-platform/.venv/bin/python"
+LOCAL_VENV="$MARKETPLACE_ROOT/mcp-servers/data-platform/.venv/bin/python"
 
-if [[ ! -f "$VENV_PATH" ]]; then
+# Check cache first (preferred), then local
+if [[ -f "$CACHE_VENV" ]]; then
+    VENV_PATH="$CACHE_VENV"
+elif [[ -f "$LOCAL_VENV" ]]; then
+    VENV_PATH="$LOCAL_VENV"
+else
     echo "$PREFIX MCP venv missing - run /initial-setup or setup.sh"
     exit 0
 fi