Reference: multi-agent collaboration & external ACP integration customization (PR #2139)

## Overview

This issue accompanies PR #2139, which is a **reference submission** rather than a request for a blind merge. It presents a complete, final-state snapshot of customizations built on BitFun's latest \main\, focusing on **multi-agent collaboration** and **connecting BitFun to external AI agents** through the ACP protocol.

## Motivation

BitFun already supports single-agent coding workflows well. This work explores two extensions that we believe are valuable directions for the project:

1. **External agent integration via ACP** — letting BitFun drive real external agent processes (CodeBuddy, Claude Code, OpenCode, etc.) as first-class sessions.
2. **Structured multi-agent collaboration** — a three-branch (decision / execution / review) coordination model with quality gates, giving large agent runs auditability and determinism.

## What the PR contains

- A **370-file final-state code snapshot** relative to upstream \main\ (no intermediate steps, no scratch files, no internal records).
- **CUSTOMIZATIONS.md**, an academic-style report: Abstract, Introduction, Related Work, Design (Three-Branch Separation-of-Powers coordination model), Implementation 1-8, Evaluation, Conclusion & Future Work, References.

## Domains covered

1. ACP channel (session-level direct connection to external ACP agents)
2. SessionControl / SessionMessage tools (create, talk to, compact, delete sessions)
3. Warden guard system (governance: repeated-failure detection, challenges, violation records)
4. RBAC subagent roles (role templates controlling subagent tool access)
5. Engine & context injection (per-round runtime facts, once-per-generation user context)
6. Legion / Task / Plan toolchain (orchestration topology, task dual lifecycle, plan tools)
7. CodeBuddy provider adapter (OpenAI-compatible cloud API adapter)
8. Web UI (flow-chat display, legion pages, model switching)

## How to review

- The PR is intentionally large (370 files) because the purpose is to present the **final state of a coherent effort** for reference.
- Maintainers may review as a whole, cherry-pick per domain, or ask to split into smaller PRs.
- Every file reference in CUSTOMIZATIONS.md was cross-checked against the final state (13/13 verified); no proprietary or internal information is included (17-pattern scan clean).

## Notes

- AI-assisted: this PR was prepared with AI assistance; the snapshot is the final state of the customization workspace.
- Testing: the snapshot compiles as part of the customization workspace; focused verification was done at workspace level.
- Related fork issue: 1688mengdie/BitFun#191

Start with PR #2139 and the referenced CUSTOMIZATIONS.md report, then review the listed ACP, SessionControl, Warden, RBAC, Legion, provider, and Web UI areas. The payload describes a 370-file reference snapshot rather than a bounded change; done would require a maintainer decision to review it whole, cherry-pick domains, or request smaller PRs.

Vom Indexierungsmodell aus dem Issue-Text verfasst.