docs: a release procedure from a green main to tagged, packaged artifacts (closes #387)

Add docs/RELEASE.md, linked from README.md's Release Artifacts section, giving
the release procedure as a numbered list: confirm main is green in CI, confirm
the one version in package.json and the two manifests matches the intended tag,
make package from a clean checkout, verify SHA256SUMS, create the annotated tag
vX.Y.Z, then distribute per browser. Each step names who performs it, marks the
owner-only ones, and states the check that it worked. Every repo command cited
(make setup, make check, make package) exists on next; tagging and verification
use standard git and coreutils, and the CRX pack line is README's own.

The per-browser distribution step is written as pending the owner's choice on
issue 386, with the Firefox and Chrome options named but none presented as
settled. Docs only: no code or test changes.

Model: opus-4-8

docs/RELEASE.md

@@ -0,0 +1,87 @@
+# Releasing AutistMask
+
+This is the procedure that turns a green `main` into a tagged, packaged release.
+It gathers into one place what is otherwise spread across the `Makefile` and
+three `README.md` sections, so the person cutting a release does not have to
+reconstruct the order from them.
+
+There is one version, declared in three files (`package.json`,
+`manifest/chrome.json`, `manifest/firefox.json`), and `make package` builds and
+packages but publishes nothing. `make build` and `make package` can be run by
+anyone; tagging, signing, packing a CRX and any upload need credentials only the
+owner ([@sneak](https://sneak.berlin)) holds and are marked **owner-only**
+below. Releases are tagged from `main` (see the Workflow section of `TODO.md`),
+so the "release commit" throughout is the `main` commit the milestone PR merged.
+
+## Procedure
+
+1. **Confirm `main` is green in CI.** The `check` workflow
+   (`.gitea/workflows/check.yml`) runs `script/cibuild`, i.e. `docker build .`,
+   and the `Dockerfile` runs `make check` as a build step, so a green `check`
+   run is a green `make check`. Find the run for the exact release commit on the
+   tracker's Actions view. _Check:_ that commit's `check` run succeeded; running
+   `make check` on a clean checkout of the commit reproduces it and exits 0.
+
+2. **Confirm the version matches the intended tag.** `package.json`,
+   `manifest/chrome.json` and `manifest/firefox.json` must all declare the same
+   `X.Y.Z`. `make build` fails when they disagree, but nothing checks that they
+   equal the tag you mean to create — that is this manual step. _Check:_ all
+   three files read the same `X.Y.Z`, and it is the version you intend to tag
+   `vX.Y.Z`.
+
+3. **Build and package from a clean checkout of that commit.** From a fresh
+   clone, or a working tree with no local modifications (`git status` clean),
+   checked out at the release commit: run `make setup`, then `make package`.
+   `make package` runs `make build` first, so the archives can only be made from
+   a `dist/` verified against that build's own receipt as a release (not debug)
+   build. It writes three files into `release/`:
+   `autistmask-chrome-<version>.zip`, `autistmask-firefox-<version>.xpi`, and
+   `SHA256SUMS`. _Check:_ those three files exist and `<version>` in the archive
+   names is the version confirmed in step 2. The Firefox `.xpi` is **unsigned**
+   (see step 6 and "Installing on Firefox" in `README.md`).
+
+4. **Verify `SHA256SUMS`.** The archives are deterministic — sorted entries,
+   fixed timestamps, fixed compression — so a second `make package` from another
+   clean checkout of the same commit produces byte-identical files. Verify the
+   recorded digests against the files with `sha256sum -c SHA256SUMS`, run from
+   `release/`. To confirm reproducibility, run `make package` again on a
+   separate clean checkout and compare the digests. _Check:_ `sha256sum -c`
+   reports `OK` for every file, and an independent build's digests match.
+
+5. **Tag the release commit.** _(owner-only)_ Create an annotated tag `vX.Y.Z`
+   on the release commit and push it: `git tag -a vX.Y.Z` (with a message), then
+   `git push origin vX.Y.Z`. _Check:_ `git tag` lists `vX.Y.Z`, and
+   `git rev-parse vX.Y.Z^{commit}` resolves to the release commit.
+
+6. **Distribute per browser.** _(owner-only; pending the owner's choice on
+   https://git.eeqj.de/sneak/AutistMask/issues/386)_ How 1.0.0 is distributed on
+   each browser is not yet decided; it is the open question on that issue, and
+   the concrete steps cannot be written until the owner records a choice there.
+   These steps need credentials only the owner holds. The options under
+   consideration are:
+    - **Firefox** — the packaged `.xpi` is unsigned, and release Firefox and ESR
+      refuse an unsigned add-on:
+        - (a) AMO self-distribution signing (unlisted): submit the `.xpi` to AMO
+          with the owner's credentials; AMO returns a signed `.xpi` installable
+          on every Firefox, with nothing listed publicly.
+        - (b) AMO listed: as (a), plus a public AMO listing and review.
+        - (c) Ship the unsigned `.xpi` and state that Firefox support means
+          Developer Edition, Nightly, or an Unbranded build with
+          `xpinstall.signatures.required` set to `false`.
+    - **Chrome** — the repo packs no CRX and publishes nothing; the extension id
+      is fixed by the `key` in `manifest/chrome.json`:
+        - (a) Chrome Web Store (unlisted): upload the `.zip` with the owner's
+          developer account; the store delivers installs and updates.
+        - (b) Self-hosted CRX signed with the private key the owner holds
+          (`chrome --pack-extension=dist/chrome --pack-extension-key=<path to the .pem>`),
+          installable only via enterprise policy on Windows and macOS, so
+          realistically Linux-only.
+        - (c) "Load unpacked" from `dist/chrome/` only, as today.
+
+    Once the owner decides, the chosen steps — including which credentials they
+    need and who holds them — are written into this section and `README.md`'s
+    installation sections are updated to match, which is part of the definition
+    of done of https://git.eeqj.de/sneak/AutistMask/issues/386. _Check:_ for a
+    store or AMO route, the artifact installs from the store or AMO on a clean
+    browser profile; for the CRX or unpacked route, the documented load succeeds
+    and Chrome reports the extension id `gipbhkogfopeahplcjhipkgpcimdpkip`.