fix(ci): environment-portable CLI tests (sys.executable -m copeca), r…

…ealistic perf bound, ANSI-proof --help assert (#6)

Six tests passed locally but failed on a fresh CI runner: 3 hardcoded .venv/bin/copeca (absent in CI), the validate perf gate assumed under 500ms on fast hardware (mostly fixed import/startup cost; 853ms on CI), and the --sign-key help assertion broke on Rich ANSI codes. Add copeca/__main__.py for python -m copeca; invoke the CLI via sys.executable -m copeca; loosen perf gate to 3000ms; disable color + strip ANSI in the help assert. Local: ruff clean + 637 passed.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>