Add configurable subagent live widget placement

[hugotown]

Summary

• Add a new ui.widgetPlacement option for async/background subagent widgets.
• Add ui.slashLiveResult = "widget" so /run, /chain, /parallel, and /run-chain can show live progress below the editor while keeping the final result in conversation history.
• Document the new ui configuration and cover async widget plus slash live widget behavior with integration tests.

Tests

• npm run test:integration ✅ (367 passing)
• node --experimental-transform-types --import ./test/support/register-loader.mjs --test test/integration/render-widget.test.ts test/integration/slash-commands.test.ts ✅ (41 passing)
• npm run test:unit ⚠️ one environment-sensitive existing failure locally: test/unit/intercom-bridge.test.ts expects pi-intercom to be missing, but this machine has pi-intercom installed globally/configured. No changed files are in that area.

[nicobailon]

This looks technically reasonable, but I would keep it draft until we decide the config surface.

The question is whether ui.widgetPlacement and ui.slashLiveResult should become supported subagent config, or whether live UI placement should stay fixed for now. I did not find another open PR covering that API shape.

[nicobailon]

I would not merge this as-is. The feature still looks plausible, but the branch now conflicts with current main in src/shared/types.ts, and the PR is still draft around whether ui.widgetPlacement and ui.slashLiveResult should be supported config.

Please rebase on current main and narrow the next revision to the config shape we want to support. One small thing to settle while doing that: the docs list /run, /chain, /parallel, and /run-chain, but the implementation also applies widget mode to /subagents-doctor.

[nicobailon]

I'm closing this for now while we clean up the PR queue. The idea may still be worth revisiting, but this branch needs a current-main revision before it can be merged. Please feel free to open a fresh revised PR later if you want to pick it back up.