fix(browse): portable temp paths — TEMP_DIRS allowlist, tmpdir()-based test files

Local path validation now accepts os.tmpdir() alongside the classic /tmp
(new TEMP_DIRS in platform.ts): on macOS os.tmpdir() is /var/folders/...,
and TMPDIR-honoring CI/sandbox environments point it elsewhere entirely —
both are legitimate scratch space. Remote file serving (TEMP_ONLY) stays
pinned to TEMP_DIR alone; no change to the exfil boundary.

commands.test.ts drops 41 hardcoded /tmp literals for a tmpp() helper on
os.tmpdir() (two message assertions now reference the same variable), and
path-validation's symlink-escape test targets /etc/hosts instead of
/etc/crontab — the target must EXIST for realpath to resolve the link (a
dangling target falls back to the link's own path and passes vacuously),
and /etc/crontab is absent on Amazon Linux.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

browse/test/path-validation.test.ts

@@ -115,7 +115,11 @@ describe('validateOutputPath — symlink resolution', () => {
   it('blocks symlink inside /tmp pointing outside safe dirs', () => {
     const linkPath = join(tmpdir(), 'test-output-symlink-' + Date.now() + '.png');
     try {
-      symlinkSync('/etc/crontab', linkPath);
+      // /etc/hosts, not /etc/crontab: the target must EXIST for realpath to
+      // resolve the link (a dangling target falls back to the link's own
+      // path, which is inside tmp and passes) — and /etc/crontab is absent
+      // on Amazon Linux while /etc/hosts exists everywhere.
+      symlinkSync('/etc/hosts', linkPath);
       expect(() => validateOutputPath(linkPath)).toThrow(/Path must be within/);
     } finally {
       try { unlinkSync(linkPath); } catch {}