Implement Proof-of-Concept Thresholds

[ShadowBearVR]

While scanning all identified potential vulnerabilities is always going to be supported, this may not be the best default behavior in terms of both time and token usage.

This PR introduces:

1. Configurable PoC Threshold (minPocSeverity): Adds the ability to limit PoC synthesis to a severity threshold (defaulting to Medium and higher), configurable per-project and per-job in Nix job specs.
2. Refined Worktree Verification: Loosens verification gate regexes to recognize passing exploit assertions and inline Rust harness test executions (otherwise all PoCs are shown as "attempted" only).
3. Review History Optimization: Skips re-evaluating findings in reviewer passes if they already have an existing review lifecycle recorded (useful for re-processing older vulnerabilities under an expanded pipeline).