v0.18.18

Fix source-free project auto-activation.\n\n- Project shell autoload now refreshes the codex alias even when CODEXPLAIN_PROJECT_DIR is already set.\n- This makes new zsh sessions inside the project route codex to .codexplain/bin/codex without source ./.codexplain/activate.\n- Keeps the previous alias restoration behavior when leaving the project.\n\nValidation:\n- cargo fmt --check\n- cargo test: 127 passed\n- codexplain quality-check --width 88: pass\n- zsh -ic cd-project check: codex alias resolves to .codexplain/bin/codex\n\nStorage note:\n- cleaned target/debug; only release binary target remains.