fix: use host-safe default AGENT_DATA_DIR for local runs

[Congregalis]

Summary

• Make AGENT_DATA_DIR default environment-aware:
  • local (non-container): ~/.clawith/data/agents
  • container runtime: /data/agents (unchanged)
• Update .env.example default behavior note
• Fixes Make AGENT_DATA_DIR default cross-platform safe (avoid /data/agents on non-container local runs) #57

Why

The previous hardcoded default (/data/agents) is container-oriented and can fail in local host environments (especially macOS), where /data may be unavailable or read-only. This led to runtime failures when endpoints attempted to create directories under AGENT_DATA_DIR.

This change keeps Docker behavior intact while making local defaults writable and cross-platform safe.

Changes

• backend/app/config.py
  • Added _running_in_container() for best-effort container detection
  • Added _default_agent_data_dir()
  • Changed AGENT_DATA_DIR default from hardcoded /data/agents to _default_agent_data_dir()
• .env.example
  • Clarified default data directory behavior for local vs container runs

Validation

• Ran python -m compileall backend/app/config.py
• Verified local default resolves to ~/.clawith/data/agents when AGENT_DATA_DIR is unset
• Verified explicit override still works (AGENT_DATA_DIR=/tmp/clawith-agents)
• Confirmed Docker-target default path remains /data/agents under container detection