feat: mandatory decomposition gate for large issues (Closes #579)

[Da-Mikey]

Automated evolution PR for issue #579. Adds evolution_decomposition_gate.py — a blocking gate that checks child issues exist before implementing issues with effort_score >= 0.4. Prevents oversized monolithic picks from reaching PR stage without decomposition.

[Lexus2016]

Integration review: blocked from merge. The PR adds scripts/evolution_decomposition_gate.py and tests, but no existing skill or pipeline step invokes it. The evolution-implementation SKILL.md still performs its own manual size/needs-split checks and never calls this gate, so the new code is dead code — it cannot enforce the policy it claims to add.

Rework brief:

• Wire the gate into skills/evolution/evolution-implementation/SKILL.md as a mandatory pre-selection check (step 2 or 2a): call "python ${HERMES_HOME}/scripts/evolution_decomposition_gate.py check --effort " and skip the issue when it exits 1.
• Make the gate consume the analysis JSON to obtain the effort_score automatically, rather than requiring a manual --effort argument in the skill path.
• Keep the existing tests; add one test verifying the CLI is documented in the skill text if feasible.

Re-open a PR once the gate is actually exercised by the implementation pipeline.