hasMarkerBlock previously swallowed every read/open error and returned
false, so an unreadable AGENTS.md (EACCES, EMFILE, EISDIR, ...) made
detectLegacyCodexSync report a clean Codex home instead of an
indeterminate inspection result. The fallback path could then skip
legacy cleanup and exit 0 with legacy artifacts still in place.
Restrict the catch to ENOENT (a missing file legitimately means no
marker block) and rethrow everything else. detectLegacyCodexSync
already propagates from hasMarkerBlock, so callers now see the actual
inspection error instead of a misleading 'no marker'.
Regression test in tests/lib/codex-legacy-sync.test.js makes a
detectLegacyCodexSync call against an unreadable AGENTS.md and asserts
that it throws something other than ENOENT, plus a sanity check that
a missing AGENTS.md still reads as no-marker.
