Merge sessions across all detected agents

[Dodhon]

Summary

Handle multiple agent session paths instead of selecting a single preferred agent.

Problem

Auto-detect currently picks the first preferred agent (main/main2) and ignores other agents, so sessions from other agents are not visible.

Scope

• Detect all agent session paths under ~/.openclaw/agents/*/sessions(.json).
• Merge sessions across agents with agent labels.
• Avoid duplicate session IDs if multiple agents share ids.

Acceptance Criteria

• Sessions list includes entries from all detected agents.
• Each session is labeled with its agent name.
• When agents share session IDs, the UI still distinguishes them (e.g., compound id or namespace).