fix(vscode): improve subagent transcript performance

[marius-kilocode]

What Problem This Solves

Live subagent transcripts in sidebar-opened editor tabs and Agent Manager became slow with many parallel agents. Unrelated child updates could reload the selected inspector, and shared-board tool output was rendered even while collapsed.

Why This Change Was Made

The selected Agent Manager child now reacts only to selection changes. Opening many child tabs is batched, cached child transcripts reconcile instead of replacing, and unchanged close-state updates do not invalidate the reactive map. Collapsed board output defers Markdown rendering and skips duplicate formatted-output work.

User Impact

Switching and following live subagent traces stays responsive with many agents. Reading older output preserves scroll position during incoming updates. Sidebar editor tabs and Agent Manager continue to share one kilo serve backend.

Evidence

Scenario	Before	After	Result
Agent Manager, 24-agent replay script time	1.12 s	0.16 s	86% lower
Editor tab, swarm update replay script time	315 ms	100 ms	68% lower
Warm child-tab switch	repeated replace loads	~30 ms	cached reconcile, parent focus preserved
Selected inspector loads during sibling activity	26 reloads	1 initial load	no unrelated reloads
Backend processes for sidebar tabs and Agent Manager	multiple surfaces	1 kilo serve	shared backend confirmed

Focused tests: 108 passed. Typecheck, lint, knip, and Kilo change checks passed. Manual self-test verified live scrolling, resume-follow, board expansion, multiple sidebar editor tabs, and one shared backend process.