1. Remove PUBLIC skills and introduce SYSTEM skills embedded in the
binary and installed into $CODEX_HOME/skills/.system at startup.
2. Skills are now always enabled (feature flag removed).
3. Update skills/list to accept forceReload and plumb it through (not
used by clients yet).
The problem is that the `tokio` task own an `Arc` reference of the
session and that this task only exit with the broadcast channel get
closed. But this never get closed if the session is not dropped. So it's
a snake biting his tail basically
The most notable result was that non of the `Drop` implementation were
triggered (temporary files, shell snapshots, session cleaning etc etc)
when closing the session (through a `/new` for example)
The fix is just to weaken the `Arc` and upgrade it on the fly
