A moved chat's finished task has no plan work page without its plan database #1984

What happened

On dev at cf282ddc6, and on #1944, a chat that moved to another computer can arrive with:

• a finished run;
• its worker and trajectory files;
• no plan database.

The finished task's summary card opens from /task, but its plan work page (description, steps, Work view) is
unavailable on the receiving computer. #1944 restores the worker transcript in this state; the plan page is still
missing.

An empty sidebar for completed-only work is expected: finished runs live in Tasks.

Replication

Deterministic (no model). On dev after #1944:

CODEAF_TELEMETRY=off go test ./internal/session -run '^TestAMovedRunKeepsItsRosterAndWorkerTranscript$' -count=1 -v

The fixture builds a chat with one settled run, no plan nodes, a worker journal, a valid trajectory step, and no plan
database. The transcript assertions pass, but PlanTasks() is empty and PlanTaskPage("t-1") returns false.

Field. Finish a proposed task (propose_task), move its chat to another computer, and open the task from bare
/task. Compare it with the same task on the sending computer.

Where

• Agent.PlanTasks, Agent.PlanTaskPage, openPlanReadHandles and planTrajectory in
  internal/session/plandb_tasks.go.
• The plan-page route in internal/tui3/taskplan.go and internal/tui3/planroom.go.

The fix

Either carry the completed plan database with the chat, or render a read-only historical page from the run receipt
and the transferred trajectory. Historical actions stay disabled, and a settled run never restarts.

Acceptance

• Through the public continue path on a keyless loopback relay:

  • finish and transfer a scripted task;
  • open it from Tasks on the receiving computer;
  • assert done, the task title, its worker transcript, and the recorded step.

  No provider call is made.

• The manual (internal/manual/chat/worker-harness.md) states the final guarantee.