fix: harden agent files API error handling to prevent React crash

[chubes4]

Summary

• Wraps list_agent_files and get_agent_file REST handlers in try/catch (\Throwable) so uncaught exceptions (e.g. missing DB tables during scaffolding) return proper WP_Error responses instead of raw 500s
• Fixes useAgentFiles query select function to always return an array via Array.isArray() — previously, API error objects ({ success: false, data: null }) passed through the nullish coalescing chain as truthy non-arrays, causing .filter() crashes
• Hardens .filter() call in handleCreateFile and hasFiles check in AgentApp with Array.isArray() guards

Bug

When loading the Agent admin page, if the /files/agent or /files/agent/SOUL.md endpoints return 500 errors, the React UI crashes with:

TypeError: a.filter is not a function
    at st (agent-react.js:1:45153)

The root cause is a cascading failure:

1. PHP handlers throw uncaught exceptions → raw 500
2. API client catches the error and returns { success: false, data: null }
3. Query select: (response) => response?.data ?? response ?? [] evaluates to the error object (truthy), not []
4. Component calls .filter() on an object → crash

Testing

1. Trigger a 500 on /files/agent (e.g. missing agents table)
2. Navigate to Agent admin page → should show "Failed to load files" instead of crashing
3. Normal operation with working API → file list loads as before