Import fp-check plugin from skills-internal

[dguido]

Summary

• Import the fp-check plugin for systematic false positive verification of security bugs
• Plugin includes a skill with standard/deep verification paths, 3 agents (data-flow-analyzer, exploitability-verifier, poc-builder), 6 reference files, and Stop/SubagentStop hooks
• Registered in marketplace.json, CODEOWNERS (@ahpaleus @dguido), and README.md

Test plan

• Install plugin locally and verify skill triggers on "is this a false positive?" prompts
• Verify agents spawn correctly during deep verification path
• Verify hooks block premature stop without completed gate reviews

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