v0.18.17

Make Codexplain slash control reversible without OOM-prone auto-builds.\n\n- /codexplain off now disables Codexplain UX while preserving the native slash bridge and local shim.\n- /codexplain on restores guidance and ansi/semantic settings after an off state.\n- Project-local patched Codex is used for native slash even when color UX is disabled.\n- TUI auto-build is disabled by default; set CODEXPLAIN_TUI_AUTO_BUILD=1 only when explicitly wanted.\n- Shim opportunistically cleans non-essential patched-Codex Cargo target cache on launch.\n- Shell auto-activation permission failures are warnings, not slash-control failures.\n\nValidation:\n- cargo fmt --check\n- cargo test: 127 passed\n- codexplain quality-check --width 88: pass\n- manual slash on/off/status: shim preserved, UX toggles correctly\n\nStorage after cleanup in the development project:\n- .codexplain: ~629M including the persisted patched Codex binary\n- target: ~3.1M release binary only\n- free space: ~24GiB