chore: remove pi-memory package

Graphiti (via pi-graphiti) is the memory layer going forward; pi-memory was
not installed anywhere and its per-worktree model downloads were recurring
test friction. Removes the package, its model-fetch script, the CI/release
cache steps, and the vitest perf-test exclude. MIGRATION.md left as
historical record.

Note: @parke.dev/pi-memory remains published on npm; Circle's wrapper may
still consume it and should migrate before the package is deprecated there.