feat(#751): add datamachine/read-debug-log ability for PHP diagnostics

[saraichinwag]

Summary

Adds ability for the system agent to read wp-content/debug.log for PHP-level error diagnostics. Enables self-debugging of fatal errors, warnings, notices, and deprecation warnings.

New Ability: datamachine/read-debug-log

Input:

• lines — Number of lines to read from end of file (default: 100, max: 1000)
• level — Filter by PHP error level: error, warning, notice, deprecated, fatal, parse, all
• since — ISO datetime filter — entries after this time
• search — Free-text search in log messages
• context — Lines of context around each match (default: 0)

Output:

{
  "success": true,
  "file": "/var/www/wp-content/debug.log",
  "entries": [
    {
      "raw": "[09-Mar-2026 14:30:00 UTC] PHP Fatal error: ...",
      "timestamp": 1741530600,
      "datetime": "2026-03-09T14:30:00+00:00",
      "level": "FATAL",
      "message": "Uncaught Exception in /path/to/file.php",
      "file": "/path/to/file.php",
      "line": 42
    }
  ],
  "total": 1,
  "filtered": 1,
  "file_size": 12345,
  "last_modified": "2026-03-09T14:30:00+00:00"
}

Use Cases

1. Self-debugging — Agent reads debug.log to diagnose PHP-level issues
2. Issue filing — Agent identifies plugin causing errors and files issue in correct repo
3. Proactive monitoring — Scheduled flow checks for new fatal errors and alerts

Changes

• inc/Abilities/LogAbilities.php — add readDebugLog, tailDebugLog, parseDebugLogLine, normalizeLogLevel
• tests/Unit/Abilities/LogAbilitiesTest.php — 5 new tests for debug log parsing

Testing

14/16 tests pass. 2 pre-existing failures (ClearLogs tests) unchanged.

Related to #751