Add Modrinth and CurseForge API publishing helper

docs/PUBLISHING_APIS.md

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+# Publishing Rivencraft with the Modrinth and CurseForge APIs
+
+The local helper is [`scripts/publish_mod_sites.py`](../scripts/publish_mod_sites.py).
+It previews a release unless `--publish` is explicitly supplied. The current
+configuration is for the Minecraft **26.3** artifact only. The 1.21.1 line needs
+its required external dependencies mapped on both sites before API publishing
+is enabled for it.
+
+## One-time token setup
+
+Create a [Modrinth personal access token](https://docs.modrinth.com/api/#authentication)
+in account settings with `VERSION_CREATE` (upload) and `USER_READ` (read-only
+credential check) scopes. Create a
+[CurseForge Upload API token](https://support.curseforge.com/support/solutions/articles/9000197321)
+from the author's API Tokens page. Treat both as passwords: never put them in
+git, chat, a release note, or a command-line argument.
+
+In a terminal at the repository root, run these one at a time and paste the
+corresponding token into the hidden prompt:
+
+```sh
+python3 scripts/publish_mod_sites.py store-token modrinth
+python3 scripts/publish_mod_sites.py store-token curseforge
+```
+
+The tokens are stored in the desktop Secret Service keyring under
+`application=rivencraft-publishing`, separately by site. The script also accepts
+`RIVENCRAFT_MODRINTH_TOKEN` or `RIVENCRAFT_CURSEFORGE_TOKEN` environment variables,
+but these take precedence over keyring entries and should not be put in shell
+history or startup files.
+
+Verify the credentials without uploading anything:
+
+```sh
+python3 scripts/publish_mod_sites.py check-token modrinth
+python3 scripts/publish_mod_sites.py check-token curseforge
+```
+
+## Release workflow
+
+Build the JAR, write a Markdown changelog, and preview the metadata:
+
+```sh
+python3 scripts/publish_mod_sites.py release \
+  --jar ports/mc263/build/libs/rivencraft-26.3-1.0.1.32-beta.1.jar \
+  --changelog ports/mc263/RELEASE_NOTES_1.0.1.32-beta.1.md \
+  --version 1.0.1.32-beta.1 \
+  --name 'Rivencraft 1.0.1.32-beta.1 — NeoForge 26.3' \
+  --minecraft 26.3 --java 25 --channel beta
+```
+
+For a *new* version only, append `--publish` to upload to both sites. Use
+`--site modrinth` or `--site curseforge` to target one site. Modrinth rejects an
+existing version number; CurseForge may not, so check its file list before any
+retry after a partial failure. CurseForge approval may take additional time.
+
+The helper uses Modrinth's documented multipart
+[`POST /version`](https://docs.modrinth.com/api/operations/createversion/) and
+CurseForge's documented
+[`POST /api/projects/{projectId}/upload-file`](https://support.curseforge.com/support/solutions/articles/9000197321)
+with the JAR, changelog, version channel, NeoForge, Minecraft version, Java
+version, and client/server environment. It sets no external dependencies for
+26.3, which uses Rivencraft's first-party Age backend.