test: first-Linux-run environment fixes — bun-only PATH shim, claude gate, darwin-scoped pdf gates

Three environmental assumptions the Linux lane exposed:
(1) gbrain-detect's deterministic SAFE_PATH lacked the bun runtime, so
every env-shebang spawn exited 127 on CI; a scratch dir holding ONLY a
bun symlink joins the PATH (appending bun's real dir would leak its
siblings — dev boxes keep gbrain there too).
(2) host-config's 'detect finds claude' assumed a claude binary; the
secretless lane deliberately has none — gated on Bun.which.
(3) The four make-pdf render gates hard-required prerequisites on ANY
CI, but the make-pdf gate workflow is macOS-only by decision and the
Linux lane doesn't build dist/pdf — hard-require scoped to darwin.

make-pdf/test/e2e/format-gate.test.ts

@@ -122,7 +122,10 @@ describe("output format gate", () => {
 
   if (!avail.ok) {
     test("format gate prerequisites are present (hard-required in CI)", () => {
-      if (process.env.CI) {
+      // Hard-require only where the binary is expected: the make-pdf gate
+      // workflow is macOS-only (path-filtered) and builds dist/pdf first.
+      // The Linux free lane deliberately doesn't build it — warn-skip there.
+      if (process.env.CI && process.platform === 'darwin') {
         throw new Error(`format gate prerequisites missing in CI: ${avail.reason}`);
       }
       console.warn(`[skip] ${avail.reason}`);