Add structured error codes to tool failure responses

[patmakesapps]

Problem

When a tool fails, the error message is sometimes just a raw {"error": "..."} string. This makes it harder for the model to recover gracefully.

Proposed solution

• Define a standard error response format with error codes, categories, and human-readable messages.
• Apply this format consistently across all tools.

Implementation notes

• Create an ErrorCode enum in core/errors.py.
• Define categories like FILE_NOT_FOUND, PERMISSION_DENIED, INVALID_INPUT, TIMEOUT.
• Return structured error dicts with code, message, recoverable, and suggestion fields.

Acceptance criteria

• All tool errors return a structured error dict with at least code and message.
• Error codes are documented.
• The model can distinguish between recoverable and non-recoverable errors.