feat(composer): running-subagent strip with per-agent filtering

[Legacynnn]

Summary

Adds a slim strip above the composer that lists a session's currently-running spawned subagents and lets you focus the conversation on one.

• While subagents are running (Claude Task/Agent or Codex subagent_*), the strip slides in flush on top of the composer with one chip per live subagent — each with a deterministic pixel-art sprite, identity color, and name — and slides out when they finish.
• Running detection keys off the tool call's result (mirroring AgentChildrenBlock's isRunning), so chips persist for the subagent's whole lifetime instead of vanishing when input streaming ends.
• Clicking a chip filters the thread to just that subagent's output. A banner at the top of the thread names the subagent and shows derived activity — type, tool uses, files touched, steps, and running/done — with a "Show all" control. The banner persists after the subagent finishes so the filter stays clearable.

Implementation

• src/features/composer/subagent-strip/ — extract-subagents.ts (single source of truth: extractRunningSubagents, selectSubagentBlock, summarizeSubagent), use-running-subagents.ts, pixel-avatar.tsx, index.tsx.
• src/features/conversation/state/subagent-filter-store.ts — Zustand store keyed by sessionId.
• src/features/panel/subagent-filter-banner.tsx + filtering wired into thread-viewport.tsx (transforms the source messages array so virtualization stays correct).
• Exported readAgentsStates / AgentState from subagent-tool.tsx for reuse.

Notes

• Per-token usage is not surfaced — it isn't carried on any rendered part (ToolCallPart, Task results, or Codex agentsStates), so the banner shows the activity it can count. Real token counts would need backend plumbing.

Test plan

• tsc --noEmit clean, biome check clean
• Unit tests for extract-subagents (running detection incl. the streamingStatus: done regression, Codex dedupe, selectSubagentBlock, summarizeSubagent) and a render test for the strip (collapse + chip toggle)
• Manual: spawn parallel Claude Task subagents — confirm the strip slides in/out, chips persist while running, click-to-filter + banner stats, "Show all" restores the thread
• Manual: repeat with a Codex subagent_spawn session