Document verified host context for the Codex sandbox probe

Changes: Add host-specific socket mask diagnosis and bounded inner workspace-write invocation to automation guidance; record independent evidence and QA handoff.
Validation: Approved host stat confirmed uid1000/mode0700; no-model inner sandbox exited 0 with project write/read/remove and outside write denial EROFS30. Final make check passed 72 tests and git diff --check passed. Initial verbose draft exceeded prompt budgets; compacted guidance passed the rerun. No storage task or model executed.