[codex] Fix make-pdf browse resolution

[parafallen-maker]

Summary

• Resolve browse from Codex/agents installs before older Claude install paths
• Reject executable directories when resolving the browse binary
• Avoid about:blank in make-pdf setup so the Chromium smoke check works with the browse URL allowlist
• Default generated PDFs and setup smoke-test output to /private/tmp on macOS so browse can write them
• Add a regression test for executable directories being mistaken for binaries

Root cause

On macOS, directories commonly have execute permission. resolveBrowseBin() only checked X_OK, so a directory such as ~/.agents/skills/browse could be selected as the browse binary. In Codex installs, the active browse binary also lives under ~/.agents/skills/gstack, while the previous global fallback only checked ~/.claude.

Validation

• bun test make-pdf/test
  • 75 pass, 1 skip, 0 fail
  • skip: local pdftotext is not installed; runtime PDF generation does not require it
• make-pdf/dist/pdf setup
  • browse binary resolved to ~/.agents/skills/gstack/browse/dist/browse
  • Chromium smoke check passed
  • smoke-test PDF generated under /private/tmp