release.sh: the release pipeline

Gates (clean tree, version single-source across FORGEFIRM_RELEASE /
rootfs stamp / .fw meta-version / tag, rootfs-vs-slot size with early
warning, installer-embedded pubkey must match the signing key,
factory-era fwup verification of the packed archive), then build,
pack, sign, checksum, and stage forgefirm.fw + sha256sums.txt +
forgefirm-image-glowforge.rootfs.wic.gz with the gh publish command
(--publish runs it where gh is authenticated). release.sh --dev packs
a dev-key-signed forgefirm-dev.fw from the release rootfs for the GUI
upload path. Signing keys are always passed explicitly - no defaults.
kas/README release order and the plan doc updated to match.

.gitignore

@@ -9,5 +9,8 @@
 # kas
 *.lock.yml.bak
 
+# release.sh staging output
+/release-staging/
+
 # Python bytecode
 __pycache__/