Make cross-run learning legible and deletable

[philiparxist]

The assistant already learns across runs via ExperienceStore (packages/personal-assistant/src/), but there's no user-facing surface for it — you can't see what it has generalised or forget a specific lesson.

Task: add a /experience CLI command (list + delete-by-id) and a short "What Aielia has learned" section to the package README, sourced from the existing store API.

Done when: a user can enumerate stored experience entries and delete one, with tests.

Comment here with your approach before starting.