Add internal/session.SessionStatus(wsDir) that derives a display-only
view of the workspace session lease. Pure read of .ctask/session.json
with no tmux invocation, no PID liveness, no lock acquisition, and no
mutation of lease state.
States: none | active | stale. Mode defaults to "direct" when a
pre-v0.5.3 lease lacks the field. Malformed leases surface as stale
with a diagnostic so the present-but-broken case stays visible
instead of being silently classified as none.
Reused for ctask info and ctask list in subsequent commits. Lifecycle
code continues to use the existing primitives (ReadLease, IsFresh,
InspectLease).
