Refactor: Unify agent context reads to persistent workspace (AGENT_DATA_DIR)

[Congregalis]

Summary

tool_dir (/tmp/clawith_workspaces) is effectively deprecated in current backend behavior.
Agent data writes no longer target that directory; they consistently go to AGENT_DATA_DIR/<agent_id>.
AGENT_DATA_DIR is always available because it has a configured default even when users do not set it explicitly.

Given this, keeping hardcoded reads from /tmp/clawith_workspaces in context/heartbeat creates unnecessary inconsistency. This PR removes that dependency and aligns reads with the actual write path.

What Changed

• Removed hardcoded /tmp/clawith_workspaces dependency from context assembly.
• Updated backend/app/services/agent_context.py to read agent files from the canonical persistent root only:
  AGENT_DATA_DIR/<agent_id>
• Updated backend/app/services/heartbeat.py to read HEARTBEAT.md from:
  AGENT_DATA_DIR/<agent_id>/HEARTBEAT.md

Checklist

• Tested locally
• No unrelated changes included