fix(web): handle native subagent payloads

[L1M80]

Summary

• normalize current native Antigravity subagent payloads and older payloadless invocation markers
• render all invoked subagents and distinguish define, message, and management tools
• show active, completed, canceled, interrupted, invalid, and failed states correctly
• keep untrusted subagent content rendered as escaped text

Verification

• pnpm test -- --run (338 tests)
• pnpm lint
• pnpm build
• git diff --check