feat(memory): add bounded inferred edge scoring

[sheawinkler]

Summary

• add opt-in same-project inferred_related scoring to /v1/memory/edges/backfill
• bound inference by scan limit, per-source peer limit, token posting cap, min shared terms, max candidates, and confidence thresholds
• mark inferred edges with deterministic IDs, inferred provenance, metadata, and idempotent write behavior
• expose CLI flags and public docs for dry-run/apply flows

Verification

• GOCACHE=/Volumes/wd_black/contextlattice/go-build-cache GOTMPDIR=/Volumes/wd_black/contextlattice/go-tmp go test -count=1 . (services/gateway-go)
• python3 -m py_compile scripts/agent/memory-edge-backfill
• git diff --check HEAD~1..HEAD
• scripts/agent_hooks/branch_lane_guard.sh --lane public
• scripts/agent_hooks/public_leak_guard.sh --mode all --ref HEAD --public