Fix review performance and quality issues

[yvette-carlisle]

Summary

• tighten Decodex runtime performance paths around retry planning, operator status snapshots, execution-event idempotency, and dashboard broadcasts
• harden Git/GitHub/Linear subprocess and HTTP behavior with bounded retries, timeouts, and safer error handling
• reduce release-delta artifact size while preserving signal-bearing comparisons, and surface signal caveats/watch state in the site

Verification

• cargo make check
• python3 -m py_compile scripts/github/*.py
• python3 scripts/github/test_build_release_delta.py
• python3 scripts/github/validate_signal_entry.py site/src/content/signals
• python3 scripts/github/validate_change_bundle.py artifacts/github/bundles
• git diff --check