feat(tui): list active and recent subagents in the sidebar - #46109

Issue for this PR

Closes #41249. Supersedes #4865 by @franlol (open).

Type of change

• New feature

Credit

The feature and the original implementation are @franlol's in #4865: subagents in the sidebar, click to navigate. This is that, rebased onto dev, with one change that came out of using it. In an orchestration session with hundreds of subagents, listing every one made the sidebar useless. So this shows the active subagents, plus a short, separate list of the last 10 finished ones. If @franlol revisits #4865, that split is the part worth lifting. If theirs lands, I'll close this. @giovannic96 requested the same feature in #41249 after shipping it as an external plugin, and #42369 asked for "running and optionally recently finished" subagents. The live activity text and the collapsible active list are @giovannic96's suggestions from their plugin, and they are credited as co-author on the commit.

Parent navigation and the return-to-parent keybind from #4865 are already on dev, so this adds just the lists.

What does this PR do?

Adds two sidebar sections. Select a row to open that child session.

• Subagents: in-flight subagents (pending and running), deduped by child session id so a resumed task_id renders once. Each row shows what the subagent is doing right now, taken from its newest message: the running or queued tool with its title or target (read: src/foo.ts, bash: bun test), or retrying 2. The text clears when the subagent starts a new turn, a tool fails, or it moves on to thinking or writing after a tool. The section collapses when it holds more than one row, and shows the count while collapsed.
• Recent subagents: up to 10 finished subagents, most recently active first, each marked Done or Failed with a relative time ("3m ago"). It collapses when long, like the file, LSP and MCP lists. A subagent never appears in both sections. The section is hidden when empty.

Both lists keep their collapsed state in KV (sidebar:subagents:open, sidebar:subagents:recent:open), so it holds across sessions, entering and leaving a subagent, and restarts. #51543 does the same for the MCP, LSP, Todo and Modified files sections; with both merged, sidebar_sections from #51543 can also move or hide this section by the name subagents.

Liveness comes from the child session's status, not the parent's task tool-part status. A background dispatch completes the parent's tool part the instant the child spawns, so that part reads completed while the child runs for minutes. On my machine, of the last 300 task parts, 270 were completed and zero were running, including a subagent that was executing at the time. Filtering on the parent part would leave the active list empty forever. api.state.session.status(childID) is the correct signal; idle sessions are absent from that map by design.

History is ordered by the child's latest activity: the newest of its session's update time, its newest message completion, and the parent task part's time. Session time.updated alone isn't enough, because it only moves when a prompt starts, so a subagent that ran for 20 minutes would sort by when it began.

Locale.relative moves from dialog-stash.tsx to util/locale.ts so both places share it. The stash dialog behaves the same.

How did you verify your code works?

• packages/tui: 58 sidebar tests pass, 0 fail, plus the full TUI suite (230 pass). Tests cover the active filter, dedup, the background-completed-parent-with-busy-child case, the 10-row cap, ordering by child activity rather than dispatch order (with a case where the two disagree), a subagent that moves between sections when resumed, failure detection from the child's last reply, each activity rule (including stale-label cases: a newer user message, a queued or failed tool, and reasoning after a finished tool), a rendered test that clicks the header to collapse and expand the active list, and rendered tests that collapse each list, remount, and find it still collapsed.
• Removing the cap, sorting by the parent part's time, dropping the activity boundary, disabling the collapse toggle, or writing either list's state to the wrong key each fails the tests.
• bun typecheck clean in packages/tui.
• The active list was confirmed live with a running background subagent. The history section, activity text and collapse are covered by tests and have not yet been checked in a live session.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR