Agent: preserve target denial reasons through MCP

[raelovejoy]

Summary

• return expected target-side denials, transport loss, and invalid requests as structured MCP results;
• preserve the target/execution-location context in those failures;
• keep unexpected programming/server exceptions as actual MCP errors;
• document the structured error contract.

Why

The first real Codex denial test correctly stopped at Relmote, but Codex only saw a generic "Error executing tool relmote_exec". It should be able to distinguish a target authorization denial from transport loss or a broken adapter without exposing credentials.