feat: wake exact session for TaaS completion delivery

[rjcloudsigma]

Summary

• adds taas.completion.deliver gateway RPC
• queues a completion system event only for the explicit OpenClaw session ID
• wakes that exact session through the native heartbeat runtime
• preserves existing session-ID hardening; no heuristic session resolution

Verification

• static inspection and source diff review completed
• dependency installation is incomplete in this checkout (typescript and tsx executables are unavailable), so plugin test commands could not run locally

Safety

• completion delivery requires sessionId + operationId + output
• no workspace/agent-derived fallback session identity is introduced