ci: auto-open release/vX.Y on a new minor/major line

.github/workflows/vendor.yml

@@ -442,3 +442,26 @@ jobs:
                  --notes "Static build toolchain $tag — clang/make/git built from source; bpftool/esbuild/libbpf headers re-hosted; qemu for the kernel-matrix runner."
           # --clobber so a same-version re-run refreshes assets.
           gh release upload "$tag" dist/* --clobber
+
+      - name: Open maintenance branch for a new minor/major line
+        if: steps.ver.outputs.skip != 'true' && github.ref_name == 'master'
+        run: |
+          set -euo pipefail
+          # When master cuts a NEW line (vX.Y.0), fork release/vX.Y at the same
+          # immutable commit so future back-ports have somewhere to land.
+          # Patches (vX.Y.Z, Z>0) and flavor builds (vX.Y.Z-<flavor>) don't open
+          # a line. Creating the branch is safe: the branch-creation push is
+          # ignored by the detect guard, so it won't cut a spurious release.
+          ver="${TC}"
+          case "$ver" in *-*) echo "flavor build ($ver) — no maintenance branch"; exit 0 ;; esac
+          if [ "${ver##*.}" != 0 ]; then echo "patch release ($ver) — no new line"; exit 0; fi
+          br="release/v${ver%.*}"   # release/vX.Y
+          if git ls-remote --exit-code --heads origin "$br" >/dev/null 2>&1; then
+            echo "$br already exists — leaving it"; exit 0
+          fi
+          # HEAD is the immutable lock commit the v${ver} tag points at.
+          if git push origin "HEAD:refs/heads/${br}"; then
+            echo "opened maintenance branch ${br} at $(git rev-parse --short HEAD)"
+          else
+            echo "::warning::could not open ${br} (ruleset/permissions?) — create it manually if a back-port is needed"
+          fi

README.md

@@ -90,21 +90,23 @@ real toolchain change.
 
 ### Back-patching an older line
 
-To ship a fix on an older release while `master` has moved on, branch the line
-from its tag and push fixes there:
+Every time `master` cuts a new line (`vX.Y.0`), CI **auto-opens** the
+`release/vX.Y` maintenance branch at that commit (patches and flavor builds
+don't open a line). So to ship a fix on an older release while `master` has
+moved on, just PR it into the existing branch:
 
 ```
-git branch release/v0.6 v0.6.0 && git push origin release/v0.6
-# PR the fix into release/v0.6, then merge
+# release/v0.6 already exists (opened when v0.6.0 was cut)
+# open a PR against release/v0.6, then merge (rebase)
 ```
 
-A push to `release/*` runs the same workflow, but the version is computed from
-the highest tag **reachable on that branch** — so a fix on `release/v0.6`
-(forked at `v0.6.0`) publishes `v0.6.1`, independent of whatever `master` is on.
-On `release/*` an unmarked commit defaults to a **patch** bump (not minor), so a
-hotfix can't accidentally collide with a mainline minor tag; use `[bump:minor]`
-/`[bump:major]` to override. The branch name is convention only — `release/vX.Y`
-(the minor line) is the natural granularity since patches walk it forward.
+A push to `release/*` runs the same workflow, but the version is scoped **by
+name** to that line's tags — so a fix on `release/v0.6` publishes `v0.6.1`,
+independent of whatever `master` is on. On `release/*` an unmarked commit
+defaults to a **patch** bump (not minor), so a hotfix can't accidentally collide
+with a mainline tag; use `[bump:minor]`/`[bump:major]` to override. (If a line
+predates auto-creation, fork it manually: `git branch release/vX.Y vX.Y.0 &&
+git push origin release/vX.Y`.)
 
 ### Flavored variants
 

build/versions.env

@@ -4,7 +4,9 @@
 # Toolchain bundle version. Each publish is an immutable, version-tagged
 # release `v<TOOLCHAIN_VERSION>` holding the whole set; consumers pin this one
 # number, and the cache is keyed by it (cache/yeet/toolchain/v<N>/<arch>/). CI
-# bumps it by 0.1 on every publish (which only happens when build/ changes).
+# sets it to the next semver tag it computes from the [bump:*] commit markers
+# (default minor; see the README). A release is cut only when build/ or the
+# vendor workflow changes.
 TOOLCHAIN_VERSION=0.1
 
 # Where a consumer fetches assets from: the version-tagged release. The fetch