fix(manager): reuse prewarmed ACP scope

[lbx154]

Summary

• propagate the Manager session ACP scope to default and role-specific backends
• prewarm the backend that actually performs model-based front-door classification
• trigger one-shot prewarm from TUI as Web already does
• preserve model classification for greetings; no local greeting shortcut

Measured effect

Real Copilot ACP, message 你好, after an 8-second prewarm window:

• before: 8.146s
• after: 3.937s
• ACP processes per session: 2 -> 1

Validation

• Manager scope/prewarm tests
• TUI suite
• Ruff
• release build and release identity tests
• real model-backed benchmark