Restructure README and migrate repo tooling layout

[yvette-carlisle]

Summary

• Align the README front matter, badges, and support/footer sections with the rsnap structure.
• Remove the top-level tools directory and move executable scripts under scripts/.
• Move GitHub signal checked-in data under artifacts/github/ and keep github-signal as a dev-only skill under dev/skills/.
• Update workflows, docs, Makefile tasks, and script defaults for the new layout.

Verification

• python3 -m py_compile scripts/github/.py scripts/config/.py
• python3 scripts/github/validate_change_bundle.py artifacts/github/bundles
• python3 scripts/github/validate_signal_entry.py site/src/content/signals
• python3 scripts/github/test_build_release_delta.py
• git diff --check
• cargo make decodex-checks
• stale-path grep over current authoritative docs/scripts passed

Notes

• Historical docs/plans snapshots and upstream-derived signal artifacts may still contain literal tools/... strings by design.