fix(cron): catch ValueError on os.open in lifecycle guard

[Braincramps]

Summary

Fixes the null-byte hang in cron/lifecycle_guard.py that can freeze an agent turn for minutes.

Problem

• Introduced in v0.20.0 (new file, added 2026-08-03).
• _read_referenced_script calls os.open(path) but only caught OSError.
• A path containing a NUL byte raises ValueError ("embedded null byte"), not OSError.
• When the model emits such a path in a terminal command, the guard scans it as a referenced script, the uncaught ValueError propagates, and the whole turn hangs (~17 min observed).

Fix

Catch (OSError, ValueError) and return (None, False) so a malformed path is skipped gracefully instead of stalling the turn. Line 260 is the only os.open call site in the file, so this single patch covers the guard.

Verification

• Confirmed os.open on a null-byte path previously raised ValueError; now returns gracefully.
• Only call site; confirmed there are no sibling os.open call sites.

[alt-glitch]

This was generated by AI during triage.

Duplicate of #77729: it contains the same os.open ValueError guard and also covers the remote-read binary fallback with regression tests.

[kshitijk4poor]

Closing as superseded by #80258, which fixes this whole bug class architecturally rather than per-callsite: path candidates are sanitized once at the ingestion boundary (NUL/empty/unexpandable tokens rejected before any OS call), text from any read_remote_script callback is sanitized at the recursion boundary (NUL = binary = nothing to scan; >1 MiB = fail closed), the remote fallback read is bounded at the source (head -c, so oversized binaries never cross the wire), and the public guard is total by construction — an unexpected walk failure logs and falls back to the direct-scan verdict instead of breaking every terminal command.

Your report and fix targeted a real member of this class — thank you. The per-callsite patches kept leaving sibling frames exposed (#76762 → #77703 → #77780 → #78256 each crashed one frame away from the previous fix), which is why we went with the boundary fix instead of merging the fragments individually. #80258 carries regression tests for the NUL-path, binary-callback, oversized-read, unset-HOME, and walk-crash cases plus an adversarial never-raises sweep.