fix(agents): harden structured error-code classification edge cases

[wpfleger96]

Post-merge review of #1653 surfaced several edge cases in the structured error-code seam where classification silently degrades or misfires. This PR closes them.

• Unknown structured codes are now authoritative: a finite lastErrorCode the UI doesn't recognize returns generic copy immediately instead of falling through to string-prefix matching, so a future code (e.g. a rate-limit classification) whose message happens to start with llm auth: can no longer render the relay-membership copy.
• Number.isFinite guards replace the != null checks, so NaN (from a non-numeric payload.code) is treated as no-code rather than entering the switch.
• When the code field is lost (downgrade, i64 parse failure), friendlyAgentLastError now recovers the code embedded in the message via the "Agent reported error (code N): " Display format before falling back to legacy prefixes.
• buzz-acp no longer truncates non-standard JSON-RPC errors to "unknown error": when the message field is missing or non-string, the full JSON object is preserved (e.g. {"code":-32000,"data":"quota exceeded"}). The duplicated extraction at the prompt and steer read loops is folded into one agent_error_from_json helper.
• Bare llm auth: / llm model not found: log-line promotion is restored in meaningful_agent_error_from_log with synthetic codes (-32001 / -32002), so older agent binaries that log unwrapped prefixes surface a classified error instead of the generic "harness exited with status N".
• The dead AgentError match arm in the transport-error branch of handle_prompt_result is removed by hoisting a single error_code extraction above both branches.
• Cleanups from the same review: accurate module docstring in friendlyAgentLastError.ts (the old one described the pre-fix(agents): surface provider errors structurally instead of raw dumps #1653 architecture), doc comments on AgentLogError, the promotes_unwrapped_llm_auth test asserts promotion again (matching its name), new coverage for the legacy log format, and the E2E fixture uses the new-format error string.

Related: #1653