fix(windows): .exe glob in .gitignore + .exe extension resolution in find-browse (#1554)

bun build --compile on Windows appends .exe to the output filename,
producing browse.exe instead of browse. find-browse's existsSync probe
only checked the bare path and returned null on Windows even when the
binary was correctly built. .gitignore similarly only excluded the
bare bin/gstack-global-discover path, leaving the .exe variant
tracked.

This commit:
- .gitignore: changes `bin/gstack-global-discover` →
  `bin/gstack-global-discover*` so the Windows .exe variant is ignored
- browse/src/find-browse.ts: adds isExecutable + findExecutable helpers
  that fall back to .exe/.cmd/.bat probing on Windows, mirroring the
  same helper already in make-pdf/src/browseClient.ts and pdftotext.ts

Contributed by @Mike-E-Log via #1554.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -4,7 +4,7 @@ dist/
 browse/dist/
 design/dist/
 make-pdf/dist/
-bin/gstack-global-discover
+bin/gstack-global-discover*
 .gstack/
 .claude/skills/
 .claude/scheduled_tasks.lock