fix(ui): use Pi-native child tool renderers

[tt-a1i]

Summary

• render Direct Subagent and Workflow child tool activity through Pi native ToolExecutionComponent
• retain real tool definitions and raw event payloads only in an ephemeral operator-side ledger
• keep persisted transcripts unchanged and preserve the compatibility renderer for historical artifacts
• retain current-session Workflow renderers after settlement so opening a completed child does not regress to JSON fallback

Root cause

The child transcript projection discarded the child session actual ToolDefinition and raw call/result payloads. Its closed handwritten formatter therefore rendered extension tools such as git_log as raw JSON even though the main Pi session used the tool native renderer.

Validation

• regression test first reproduced git_log rendering as raw JSON
• focused Direct/Workflow transcript tests: 23 passed, 0 failed
• bun run check
• bun run test (Node 889 passed; Vitest 30 passed)
• git diff --check

Isolated partial pi -e smoke attempts were not counted as acceptance because they did not reconstruct the canonical OpenPI tool surface. The installed stable runtime was left untouched.

Closes #105