Add TUI fleet dashboard pane (Phase C1)

[tylerreckart]

Summary

Phase C1 of the fleet dashboard: a TUI consumer of the already-shipped fleet SSE model so a user can watch and steer a multi-agent job without reading raw events.

This is not a second orchestrator. The tree subscribes to the same session/SSE callbacks the TUI already uses for streaming (stream_id + depth, plus Phase B agent.spawned / agent.teardown / reconcile.delta).

What you get

• In-memory fleet tree keyed by stream_id (and JIT clone_id). Open on stream_start / agent.spawned; update on agent_start, tool_call, token_usage, text, reconcile.delta; close on stream_end (keep until the next job) or agent.teardown (remove immediately).
• Right-rail Fleet pane (~24–28 cols, same width breakpoint as the session sidebar). Live tree with depth indent, agent name, active tool or token totals, and a ◇ marker on reconcile JIT workers. Hidden when idle so it does not drown conversation panes; stays up in multi-pane layouts. Stacks above the session box when both want the rail.
• Steer / focus: Ctrl-w f shows and enters the tree (second press hides). Click or Enter focuses the conversation pane bound to that stream (/parallel children stay on the host pane; /pane children have their own leaf). Esc on a running row maps to the existing cancel_pane_turn kill-switch — no new cancel protocol.
• Thin reconcile overlay: for mode=ensure jobs, residual vs held and the current wave sit above spawn/teardown rows.

Docs / roadmap

• New: docs/tui/fleet.md
• Cross-links in panes, keybindings, streaming, SSE catalog
• ROADMAP Phase 5 fleet line + 5-agent acceptance checked
• CHANGELOG Unreleased entry

Tests

unit_fleet covers event → tree transitions (parallel indent, depth cap 2, tool/token aggregation, JIT spawn/teardown, stream attach to clone, overlay, SSE fixtures, 5-agent paint, request_id clear). Mouse hit-test covers overlay-skipped row mapping. No live LLM.

Out of scope for this PR: C2–C4 (delegation policies, workflow recipes, TUI admitting reconcile directly), advisor policy packs, plan-to-execution observability.