fix(plugins): remove broken mcpServers references that broke plugin loading

The MCP consolidation commit (afd4c44) deleted plugin-level .mcp.json files
but left references to them in plugin.json and marketplace.json. This caused
7 plugins to fail loading (projman, pr-review, cmdb-assistant, data-platform,
viz-platform, contract-validator, and indirectly git-flow/clarity-assist).

Changes:
- Remove mcpServers field from 6 plugin.json files (file no longer exists)
- Remove mcpServers field from 6 marketplace.json entries
- Add file reference validation to validate-marketplace.sh:
  - Validates mcpServers references point to existing files
  - Validates hooks references point to existing files
  - Validates commands references point to existing paths
- Add pre-commit hook (.git/hooks/pre-commit) to enforce validation

The validation script will now FAIL if any config file references a
non-existent file, preventing this class of bug from happening again.

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

plugins/contract-validator/.claude-plugin/plugin.json

@@ -18,6 +18,5 @@
     "interfaces",
     "cross-plugin"
   ],
-  "commands": ["./commands/"],
-  "mcpServers": ["./.mcp.json"]
+  "commands": ["./commands/"]
 }