feat: structured errors

[yarolegovich]

Implements the spec structured error with a few minor adjustments:

• Collapsed A2ACLI_ERR_CARD_NOT_FOUND, A2ACLI_ERR_CARD_INVALID into a single cardResolution code, and A2ACLI_ERR_CREDENTIALS_MISSING, A2ACLI_ERR_AUTH_FAILED into a single unauthenticated code. I think this simplifies the code surface and extra branching is not justified.
• A2ACLI_ERR_UNREACHABLE - renamed to A2ACLI_ERR_IO, so that it also covers file IO failures or connection resets and can be identified more reliably than a very specific "unreachable".

Also introduced a new ERR_PROTOCOL to report and A2A protocol canonical errors.

fixes #28 #29