feat(swarm): muxa swarm — a fleet command center that dispatches work

[jiunbae]

What

Adds muxa swarm, a fullscreen TUI that steps up from muxa's per-session views (watch, dashboard) to a fleet-level command center. This is the first concrete step in the "agent + tmux helper → agent swarm manager" repositioning: the dashboard/TUIs were observation-only; swarm adds command.

Every tracked agent becomes a node in a swarm map, clustered by project (cwd) or tmux session, rendered as a state-colored grid with a live fleet-status readout (● working ▶ input ◆ choice ■ error … N need you). It wears muxa's signature violet brand accent, unifying the CLI with the BarShelf menu-bar widget.

Dispatch — you can give the swarm work

key	action
Enter / p	compose a prompt for the selected agent
b	broadcast one prompt to a marked squad (Space to mark), or to every input-blocked agent when nothing is marked
x / K	abort the current turn / terminate — behind a confirm popup
o	jump to the selected tmux pane on exit
g	toggle grouping (project ↔ session) · Tab jumps clusters · ? help

Dispatch reuses the existing trusted side effects rather than reinventing them:

• tmux send-keys via watch::dispatch_quick_action for pane agents
• IPC write_session / terminate_session for muxa-owned PTY sessions

PTY surfaces win over the legacy pane field so a pty:* agent is never driven as a tmux pane it doesn't own. No new dependencies.

Design

Pure board/target logic is factored out of the TUI so it's testable without a daemon or terminal:

• build_swarm_data — snapshot → clustered, urgency-sorted board + fleet tally
• broadcast_targets — marked squad, else the input-blocked set
• DispatchTarget::for_agent — PTY-over-pane routing

Testing

• 9 new unit tests (fleet tally/attention, paneless filtering, project grouping, most-urgent-cluster-first sort, broadcast target resolution both ways, PTY-over-pane routing, width-aware truncation, duration formatting)
• cargo test --workspace — green (unit + e2e once bins are built)
• cargo clippy --workspace --all-targets -- -D warnings — clean (incl. pedantic)
• cargo fmt --all -- --check — clean

Notes / follow-ups

• Live updates use a 1s snapshot poll (matches muxa dashboard); a push-based subscribe() stream is a natural follow-up.
• Broadcast/prompt injection is intentionally non-destructive (the composer is the deliberate step); abort/terminate keep the confirm gate.
• Next rungs on the ladder: dispatching new agents (SpawnSession), a task/goal queue, and a web-dashboard swarm map with the same violet identity.

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