Harden sed parsing for terminal auto-approval edge cases

[anthonykim1]

Follow-up to #328882.

The immediate fix reuses the existing workbench SedFileWriteParser to block canonical sed -i / --in-place forms in Agent Host. Broader parsing and destination-aware alignment should be handled separately so the initial fix stays small.

Potential follow-up areas:

• Shell-dialect-aware executable matching, quoting, and backslash handling.
• Quoted, escaped, abbreviated, and runtime-expanded option words.
• GNU versus BSD sed -i interpretation and PATH-selected implementations.
• Backup suffix destinations, including suffixes containing *.
• Glob/runtime-expanded targets and --follow-symlinks behavior.
• Destination-aware Agent Host approval for statically resolvable in-place edits, aligned with workbench behavior.
• Ensure forwarded and full-command allow rules cannot bypass ambiguous or rejected writes.

Add focused cross-platform tests before expanding the parser.