feat(desktop): enforce managed ACP runtime contracts

[vincentkoc]

Summary

• add opt-in compatibility metadata and bounded noninteractive probes for managed ACP runtimes
• verify the exact resolved executable again at the shared spawn boundary
• support CLI updates through Buzz's private managed Node/npm prefix
• propagate incompatible and indeterminate runtime states through setup payloads and Desktop UI
• preserve parent BUZZ_* context when buzz-acp starts the ACP child

This is the generic lower layer of the OpenClaw runtime stack. Existing runtimes
remain opted out until they declare a compatibility probe. The stacked OpenClaw
change is the first consumer.

Related issue

• Managed agents: add Hermes, Kimi Coding, OpenCode and OpenClaw runtime adapters #3696
• stacked upper PR: feat(desktop): add OpenClaw as a first-class ACP runtime #3868

Testing

• cargo test -p buzz-acp setup_mode (24 passed)
• cargo test -p buzz-acp spawn_inherits_parent_buzz_context (1 passed)
• cargo test compatibility in desktop/src-tauri (5 passed)
• cargo test cli_probe in desktop/src-tauri (8 passed)
• cargo test agent_discovery in desktop/src-tauri (133 passed)
• cargo fmt --check
• focused Desktop unit tests in the combined stack (125 passed)
• pnpm build:e2e in desktop
• Playwright tests/e2e/harness-management.spec.ts in the combined stack (15 passed)
• autoreview clean