CI: Per-crate gate overrides in det-policy classification system

[flyingrobots]

Context

warp-benches was promoted from DET_NONCRITICAL to DET_IMPORTANT in PR #283 as a pragmatic compromise. Ideally, the classification system should support per-crate gate overrides so a crate can declare which specific gates it requires (e.g., required_gates: [G3]) without needing to change its entire classification tier.

Current Behavior

• classify_changes.cjs only reads crateInfo.class to determine gates
• The crate-level required_gates field (removed in feat: TTD Hardening Sprint S1 — Gates and Evidence Integrity #283 as dead config) was never consumed

Desired Behavior

• det-policy.yaml supports optional crate-level required_gates that augments the class-level gates
• classify_changes.cjs merges class gates + crate-level overrides
• Example: warp-benches could be DET_NONCRITICAL with required_gates: [G3], triggering G3 when benchmarks change without triggering G1/G2/G4

Acceptance Criteria

• classify_changes.cjs reads and merges crate-level required_gates
• validate_det_policy.cjs validates crate-level gates against ALLOWED_GATES
• At least one crate uses the override (warp-benches)