Support Copilot and Qwen ACP providers

[ymichael]

Summary

• add built-in ACP provider detection for GitHub Copilot and Qwen Code
• remove Gemini from built-in ACP support after the upstream client failure
• harden unavailable/unsupported ACP provider handling
• fix ACP approval mapping so Copilot/Qwen permission prompts resolve as command approvals
• accept null ACP model descriptions from native model discovery
• update provider docs, guide templates, and bb-cli skill

Manual QA

• validated against isolated bb dev instance with real CLIs/auth
• Copilot: discovery, model list, smoke turn, follow-up continuity, full write, workspace-write approval, readonly denial
• Qwen: discovery, native model list, native model spawn, smoke turn, follow-up continuity, full write, workspace-write approval, readonly denial
• verified Gemini is absent from provider discovery and unsupported requests fail
• checked pending interaction table after fixed runs; no unresolved ACP approvals remained

Automated Validation

• pnpm exec turbo run test --filter=@bb/agent-runtime -- src/acp/adapter.test.ts src/acp/bridge/bridge.test.ts src/acp/bridge/model-catalog.test.ts
• pnpm exec turbo run typecheck --filter=@bb/agent-runtime
• pnpm exec turbo run test --filter=@bb/server -- test/system/execution-options.test.ts test/services/threads/thread-execution-plan.test.ts test/threads/thread-runtime-config.test.ts
• pnpm exec turbo run typecheck --filter=@bb/server
• pnpm exec turbo run typecheck --filter=@bb/templates
• pnpm exec turbo run test --filter=@bb/templates
• git diff --check