fix: always replay the current todo list on session/load

[william0wang]

Problem

A re-attaching client (the mobile app always re-attaches via
session/load) never learned a session's current todo list: the shared
per-session differ only emits PlanUpdate on plan CHANGE, and its
lastPlanSig was already aligned by whatever client attached first —
the second load's diff came back empty.

Fix

• Keep running the shared differ's full diff for its mark-seen side
  effect (turn completion must not re-emit replayed history), but
  discard its events.
• Emit the CURRENT todos through a throwaway differ whose
  "__none__" sentinel makes diffPlan always fire — every load now
  carries the plan, changed or not.

Testing

• tests/load-plan-replay.test.ts: a second loadSession on the same
  session still emits a PlanUpdate with the current entries. Verified to
  fail without the fix (stash → red, pop → green).
• Full suite: 689/689 passed; typecheck/lint/build clean.
• Version 0.11.5 + CHANGELOG.