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

README.md

@@ -64,7 +64,9 @@ release/SHA256SUMS
 ```
 
 Nothing is published by this. Tagging, CRX packing and any upload are
-outward-facing acts and are the owner's alone.
+outward-facing acts and are the owner's alone. The full procedure that turns a
+green `main` into a tagged, packaged release — the order of steps, who performs
+each, and how to check it worked — is in [docs/RELEASE.md](docs/RELEASE.md).
 
 The archives are deterministic — entries sorted, timestamps fixed, compression
 level fixed — so two builds of one commit produce byte-identical files and the