Agent label for custom sessions paths

[Dodhon]

Summary

Improve agent labeling when AGENT_SESSIONS_PATH points to a non-standard path.

Problem

Agent name inference relies on ~/.openclaw/agents/<agent>, so labels are missing when sessions are loaded from custom paths.

Scope

• Allow explicit AGENT_NAME override (or derive from file name where possible).
• If multiple paths are supported, allow per-path agent name mapping.

Acceptance Criteria

• Agent label is present even when AGENT_SESSIONS_PATH is a custom path.
• Documentation explains how to override the agent name.