Address re-review minors on #1487:
- bin/install.js: replace the silent .gsd-source marker catch with a
console.warn so a failed write is diagnosable (walk-up also fails on the
Claude-global layout, so a swallowed error still breaks /gsd-surface).
- tests/runtime-artifact-layout.test.cjs: add a writer fault-injection case
(mock fs.writeFileSync to throw for the marker) proving the catch branch is
reachable, install stays non-fatal, and the warning is emitted.
bc2e4395ac