feat(tooling): add locomo single-knob auto-stop gate

[Cocoblood9527]

Summary

• add tools/locomo/evaluate-single-knob.py to enforce APPLY/NO-OP gate for fixed single-knob candidates
• gate rule is hardcoded: apply only if both overall and multi-hop strictly improve
• add V2-R14 (gate) and V2-R15 (closure) spec/plan/results docs
• update LoCoMo README and root README usage

Test Plan

• ./.venv/bin/python tools/locomo/evaluate-single-knob.py --output /tmp/localmemos-v2-r14-single-knob-result.json
• cargo test --workspace
• ./.venv/bin/pytest python/tests -q
• corepack pnpm --dir packages/node test
• corepack pnpm --dir packages/mcp test
• REBUILD_PYTHON=0 tools/locomo/run-full-gate.sh 5