Add changelog-only Discord notifications after successful pushes

AGENTS.md

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+# Push changelogs
+
+The user authorized Discord changelogs after new changes are successfully pushed.
+After each successful push to `origin/main` from this checkout:
+
+1. Prepare a concise, player-facing changelog covering the commits in that push.
+   Include only the changelog: describe fixes/features accurately, without server,
+   deployment, or Git push status. Do not add operational disclaimers or footers.
+   Phrase gameplay fixes without mentioning the server (for example, "Fixed radio
+   station generation after an empty-session wait").
+2. Save the message to a temporary file and run
+   `python3 tools/discord_changelog.py --commit <pushed-sha> --message-file <path>`.
+3. Report a delivery failure honestly. Do not blindly retry a pending delivery;
+   check Discord first to avoid duplicates.
+
+The webhook credential is in `.local/discord-webhook.url` (ignored, owner-only).
+Never print it, commit it, or put it into changelogs or command-line arguments.
+The helper verifies the remote commit, disables mentions, and records delivery in
+`.local/discord-changelog-state.json`. Do not backfill old commits unless requested.
+
+This is an assistant workflow, not a server-side Git hook. Notifications for
+pushes from other checkouts require a Discord push webhook in Gitea repository
+settings; that server-side automation has not been configured.