v0.18.16

Fix Codex TUI adapter rebuild detection and project-local cache cleanup.\n\n- Prepare upstream before apply/build so missing local Codex source is recovered.\n- Detect current assistant-color patch markers to avoid reapplying an already patched TUI.\n- Persist the project-local patched Codex binary and clean the multi-GB Cargo target cache after build.\n- Keep session activation guidance stable with source ./.codexplain/activate.\n\nValidation:\n- cargo fmt --check\n- cargo test: 126 passed\n- codexplain quality-check --width 88: pass\n- codexplain tui-adapter build: project-local patched Codex TUI ready