docs: add release flow for nightly and main

docs/RELEASE.md

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+# Release Flow (Main + Nightly)
+
+This project uses two release channels:
+
+- `nightly`: integration and test channel
+- `main`: stable channel
+
+## Rules
+
+- Feature work goes to `nightly` only.
+- Promote from `nightly` to `main` with `--squash` only.
+- `main` version has no suffix (`0.1.60`).
+- `nightly` version uses `-nightly` and is always at least one patch higher than `main` (`0.1.61-nightly`).
+- Keep changelogs split:
+  - `CHANGELOG-NIGHTLY.md`
+  - `CHANGELOG.md`
+
+## Nightly publish
+
+1) Finish changes on `nightly`.
+2) Bump addon version in `addon/addon.xml` to `X.Y.Z-nightly`.
+3) Build and publish nightly repo artifacts.
+4) Push `nightly`.
+
+## Promote nightly to main
+
+```bash
+git checkout main
+git pull origin main
+git merge --squash nightly
+git commit -m "release: X.Y.Z"
+```
+
+Then:
+
+1) Set `addon/addon.xml` version to `X.Y.Z` (without `-nightly`).
+2) Build and publish main repo artifacts.
+3) Push `main`.
+4) Optional tag: `vX.Y.Z`.
+
+## Local ZIPs (separated)
+
+- Main ZIP output: `dist/local_zips/main/`
+- Nightly ZIP output: `dist/local_zips/nightly/`