feat(pi-error-log): central tool error log

New package @parke.dev/pi-error-log: hooks tool_execution_end and
appends one sanitized JSON line per failed tool execution to
~/.pi/logs/errors.jsonl, with 10MB rotation to .1. Args are deep-
redacted (sensitive keys, secret-looking values), capped at 4KB, and
circular-safe; the logger never throws. An error_log tool reads the log
newest-first with tool/kind/since/limit filters, tolerating corrupt
lines.

Note: pi's extension API has no extension_error event (extension errors
only surface via the internal ExtensionRunner.onError), so only tool
errors are captured.