feat: add per-session AutoRouter algorithm override

[rjcloudsigma]

Summary

• add taas.autorouter.setAlgorithm { sessionId, algorithm } with null clear semantics
• validate against the TaaS algorithm allowlist and keep overrides in a bounded 256-session LRU-like map
• inject X-TaaS-Autorouter-Algorithm into transport turn state and wrapped stream options
• preserve existing behavior when no override exists
• bump plugin version to 0.12.0

Studio integration contract

Call the write-scoped gateway method before the next auto-model send:

{"sessionId":"<OpenClaw session id>","algorithm":"cost"}

Allowed: best_fit, price_performance, savings_curve, cost, ttft, tps. Clear with algorithm: null. TaaS org/department enforcement retains precedence.

Verification

• npm run build
• npm test (39/39)
• npm run typecheck
• npm run smoke