docs: add Show HN launch gate checklist

[Gradata]

Summary

• Adds a PR-backed Show HN launch gate checklist artifact for GRA-2509.
• Records verified offline quickstart smoke output from origin/main checkout 2b12800.
• Separates launch-safe claims from blockers: README benchmark claims still need qualification/removal, and current telemetry does not yet measure WAU.
• Includes a caveated Show HN post draft that avoids unsupported WAU or benchmark claims.

Paperclip issue UUID: 014ba174-2314-4063-8bc2-a4dc2c3d955b
Paperclip identifier: GRA-2509

Verification

env -u BRAIN_DIR -u GRADATA_BRAIN PYTHONPATH=src python3 examples/offline_quickstart_smoke.py
# ✓ offline quickstart smoke passed

git diff --cached --check
# no output before commit

[greptile-apps]

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