Name the legacy Pi agent directory for Atomic auto auth

Atomic reads PI_CODING_AGENT_DIR when ATOMIC_CODING_AGENT_DIR is unset,
so the auto-mode auth.json lookup lists both variables in precedence order.

Assistant-model: Claude Opus 5.5
Assistant-workflow: inline
Assistant-verification: source check passed: @bastani/atomic 0.9.23 dist/config.js getEnvValue prefers ATOMIC_CODING_AGENT_DIR, then PI_CODING_AGENT_DIR, and a set variable disables the ~/.pi fallback
Assistant-verification: dprint ci passed: bun run ci
Co-authored-by: Alex Lavaee <lavaman131@github.com>