Show when a run was stopped or interrupted, and tell the agent #442

Found in the desktop tools study (#427), D3 and D3b on Canary 0.0.46+684262d. Stopping and restarting worked: nothing was replayed, and an interrupted action was left as unknown. What the timeline shows doesn't match that.

Observed:

• After the owner presses Stop during a desktop run, the timeline shows the last tool calls as ordinary results, and nothing says the run was stopped. Tester confusion 1/3.
• In the next turn, the agent doesn't know its previous run was stopped by a person. It said: "Neither run hit an error or refusal on my side… I can't see why it stopped; my guess is the run was halted from outside."
• When the channel's worker is killed while the model is still writing a tool call, the partial call (arguments without text, stopReason: aborted) shows as a red desktop_insert entry with "Stopped.". It opens to nothing. The run then resumes after the worker restarts, and its summary ("Worked for 2 minutes 8 seconds") doesn't mention the restart. Tester confusion 2/3.

Expected:

• A Stop leaves a visible marker: who stopped it, when, and which in-flight action ended as unknown.
• The agent's next turn includes a note that the previous run was stopped by its owner, or interrupted by a restart.
• A tool call that never ran is labeled as such, for example "not run: the agent was interrupted while writing this call", and a resumed run says it resumed.

Part of #427.