fix(ci): a removed layout writer no longer fails other PRs' ratchet check (#27557)

#27529 removed two main-process layout writers after the ratchet landed, so main's
baseline went stale and every PR failed the check. Prune the baseline and report a
shrunk count as a warning; only growth fails.

config/main-layout-writer-baseline.txt

@@ -3,10 +3,10 @@
 # This is a RATCHET: counts may only go DOWN. Layout moves to one runtime owner; do not add
 # writers. Prune after removing writers: pnpm check:layout-writer-ratchets --prune
 1 src/main/ipc/pty/ipc/leaf-move.ts
-2 src/main/ipc/pty/ipc/spawn-commit-persist.ts
+1 src/main/ipc/pty/ipc/spawn-commit-persist.ts
 1 src/main/ipc/pty/pane/pane-owner-replacement.ts
 3 src/main/ipc/pty/pane/stable-owner.ts
-2 src/main/ipc/pty/runtime/spawn-commit.ts
+1 src/main/ipc/pty/runtime/spawn-commit.ts
 1 src/main/ipc/renderer-shutdown-checkpoint.ts
 1 src/main/ipc/runtime-environment-removal-cleanup.ts
 1 src/main/ipc/runtime-environment-session-reconcile.ts

config/scripts/check-layout-writer-ratchets.mjs

@@ -455,12 +455,13 @@ export function main(root = process.cwd()) {
         `::error file=${file}::${now} ${ratchet.label}, baseline allows ${allowed}. ${ADVICE}`
       )
     }
+    // Why: a removal must not fail PRs that branched before it; only growth fails.
     for (const { file, now, allowed } of shrunk) {
-      console.error(
-        `::error file=${file}::${now} ${ratchet.label}, baseline still allows ${allowed}. Run: pnpm check:layout-writer-ratchets --prune`
+      console.warn(
+        `::warning file=${file}::${now} ${ratchet.label}, baseline still allows ${allowed}. Run: pnpm check:layout-writer-ratchets --prune`
       )
     }
-    if (grown.length + shrunk.length > 0) {
+    if (grown.length > 0) {
       failed = true
     } else {
       console.log(`Layout-writer ratchet (${ratchet.id}) OK — ${total(current)} ${ratchet.label}.`)