Fix resume creating new agents instead of restarting stopped ones

[ptone]

Summary

Fixes ptone#61 — scion resume was creating new agents instead of resuming stopped ones.

• Root cause: The Hub server's CreateAgentRequest struct was missing the Resume field that the CLI client sends. All stopped agents were treated as "stale" and deleted+recreated, losing the agent ID, template association, and metadata.
• Fix: Added the Resume field to the Hub's CreateAgentRequest and added a new handler path in handleExistingAgent that restarts stopped agents in-place when Resume=true, mirroring the existing suspended-agent path.
• Preserved behavior: scion start on a stopped agent (without Resume) still deletes and recreates — only scion resume triggers the in-place restart.

Test plan

• TestCreateAgent_ResumeFromStoppedStatus — verifies resume returns 200, preserves agent ID, calls DispatchAgentStart (not delete+create)
• TestCreateAgent_StartFromStoppedStatus_NoResume — verifies start (no resume flag) still recreates with 201
• TestCreateProjectAgent_ResumeFromStoppedStatus — same resume test through project-scoped endpoint
• Existing TestCreateAgent_RecreateFromStoppedStatus still passes (no regression)
• Full go test ./pkg/hub/... passes
• go vet ./... and go build ./... pass

[gemini-code-assist]

Code Review

This pull request fixes an issue where resuming a stopped agent created a new agent instead of resuming the existing one in-place. It introduces a Resume flag to the CreateAgentRequest and updates the agent-handling logic to preserve the agent record and restart the container when this flag is set. Comprehensive unit tests have been added to verify this behavior. The review feedback suggests defensively initializing existingAgent.AppliedConfig if it is nil to prevent potential issues when applying task and attach parameters.

[gemini-code-assist]

If existingAgent.AppliedConfig is nil, the task and attach parameters will not be updated, and subsequent calls (such as dispatcher.DispatchAgentStart or database updates) may encounter nil pointer dereferences.

To prevent panics and ensure robust handling, defensively initialize existingAgent.AppliedConfig if it is nil before applying the request parameters.

		if existingAgent.AppliedConfig == nil {
			existingAgent.AppliedConfig = &store.AgentAppliedConfig{}
		}
		if req.Task != "" {
			existingAgent.AppliedConfig.Task = req.Task
			existingAgent.AppliedConfig.Attach = req.Attach
		}