fix(ci): silence Discord on auto-release cascade ticks

[bh679]

Summary

• The auto-release cascade dispatches release.yml with auto=true but never set notify_discord=false. Since notify_discord defaults to true, every cascade tick (hourly → 5h → daily, ~22 fires over 14 days per real release) pinged the Discord server.
• Gate the Notify Discord step on !inputs.auto. The auto input is already the source of truth for "this is a cascade tick" — auto-release-reset.yml keys off it too — so reusing it keeps the policy intrinsic and consistent.
• Modrinth + CurseForge publishes still fire on every cascade tick, preserving the "recently updated" feed-freshness purpose of the cascade.
• Updates CLAUDE.md ### Discord noise (known followup) → ### Discord notifications, flipping it from "known concern" to "resolved, behavior documented".
• Updates the notify_discord workflow input description so it accurately reflects that the flag only governs manual releases now.

Behavior matrix

Dispatch	auto	notify_discord	Discord notifies?
Manual release (default)	false	true	yes
Manual release, hotfix one-off silence	false	false	no
Auto-release cascade tick	true	true (unused)	no
Force-fire cascade (force=true)	true	true (unused)	no

Test plan

• Local: python3 -c "import yaml; yaml.safe_load(open('.github/workflows/release.yml'))" — passes
• Local: ./gradlew build — passes, all tests green, jar produced
• Manual review of updated if: condition against the behavior matrix
• Post-merge: next real release continues to notify Discord
• Post-merge: first cascade tick after the next real release skips Discord (Notify Discord step shows as "skipped"), Modrinth + CurseForge publishes still fire
• Optional post-merge sanity check: gh workflow run auto-release.yml -f force=true and inspect the dispatched release.yml run for a skipped Discord step