[benbrandt]

Stacked on #66101 (linked session message views). Item 6 part 2 of the ACP subagents RFD work.

Until now an ACP parent's first message to a subagent was drawn as one card for the whole subagent, and later messages stayed "To …" rows. Native subagents get one card per spawn_agent call. This PR gives ACP subagents the same: one card per delegation.

Which messages are cards

• A message linked to the subagent's view of it gets its own card. claude-agent-acp links launches and, since v0.96.0 (feat: report SendMessage delegations as linked session messages agentclientprotocol/claude-agent-acp#1424), SendMessage delegations.
• The card covers its part of the subagent's transcript: from the linked view up to the next message from the parent.
• Without a link, the first message still gets a card for the whole subagent, and later ones stay rows. This is unchanged for agents that don't send references.

Status of earlier cards
The subagent reports one state for all its work, and the protocol has no per-message outcome. So:

• A card for an earlier message shows the first idle state the subagent reported after receiving it. That idle ended the message's work.
• Until that idle arrives, the earlier card shows the current state, like the latest card. That is the case when a message arrives while the subagent is still working on the previous one, so both are part of the same run.
• AcpThread records each idle state an ACP subagent reports, with its entry count at that time, for this (subagent_outcome_after). Replayed states are recorded in order, so earlier cards keep their outcome after a reload.
• The latest card and the child's titlebar still show the current state. Stop still needs a running subagent that advertises cancel.

Not changed: a card's full-screen button opens the subagent where it was last scrolled, as for native cards. The row link from #66101 jumps to the linked entry; doing the same here would also change native cards, so I've left it as a follow-up.

Testing

• test_each_linked_message_to_an_acp_subagent_is_its_own_card:
  • two delegations, the second arriving while the first is still running: both cards show their part and are running;
  • then a refusal and a third delegation: the first two cards show Failed and the third Running;
  • an unlinked fourth message stays a row.
• That test fails with the per-delegation slices disabled, and with earlier cards using the current state.
• test_first_message_to_an_acp_subagent_shows_its_card (unlinked messages) passes unchanged.
• cargo nextest run -p acp_thread -p agent_servers -p agent_ui -p agent -p sidebar (2021 tests, rebased on main after Update agent-client-protocol to 3.6.0 (schema 1.13) #66245), ./script/clippy and cargo fmt pass locally.
• Not tried in the running app.

Release Notes:

• N/A