v1.3.0 — Agent-IO Helper + Per-Verb Help

What changed

Audit against CLI agent principles — 7 principles, all addressed:

• NEW _tools/cex_agent_io.py — shared helper (7 functions, ~260 LOC)
• [exit:N | Xms] metadata tail + --- stderr --- separator
• truncate_or_file() — 200 lines / 50 KiB cap
• read_content() — stdin separation (the "90% of escape issues" fix)
• maybe_print_verb_help() — per-verb help
• 10 critical tools migrated, 22 per-verb help endpoints across 9 tools
• _run() in hooks_native now separates stderr on failure

Full Changelog: v1.2.0...v1.3.0