feat(harness-fx): implement fx harness adapter (#19374)

## Background

With [fx](https://fx.sh) supporting ACP, it should be supported as a
first-party harness adapter.

## Summary

This adds a sandbox-backed fx adapter on top of the shared ACP v1 layer,
preserving central ACP authentication and lifecycle behavior while
mapping fx-specific tools, permissions, and MCP namespaces.

- Add `harness-fx` with official installer-based acquisition and AI
Gateway credential brokering.
- Map fx built-in tools and permission modes, including dynamic external
MCP tool classification.
- Mirror the harness examples in `ai-functions` and `harness-e2e-next`,
including examples for known unsupported capabilities.
- Document setup, authentication, settings, and ACP limitations, and
cover installer-managed fx in harness dependency tooling.

## End-to-End Verification

Ran the added examples in `examples/ai-functions/src/harness-agent/fx`
and the interactive examples for fx in `examples/harness-e2e-next`.

## Checklist

- [x] All commits are signed (PRs with unsigned commits cannot be
merged)
- [x] Tests have been added / updated (for bug fixes / features)
- [x] Documentation has been added / updated (for bug fixes / features)
- [x] A _patch_ changeset for relevant packages has been added (for bug
fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)

## Future Work

Add native instruction and structured-output mappings if or once fx
exposes corresponding ACP extensions.