fix(desktop): make harness errors runtime-neutral

[tbedor-square2]

Why

A bare ACP -32603 Internal error does not identify the failing harness, but Buzz always showed Codex-specific troubleshooting. That sent users of Amp and other ACP runtimes toward unrelated configuration.

What

• Replace the Codex-specific fallback with harness-neutral runtime-log guidance
• Preserve adapter-provided details and add a regression that generic errors never mention Codex

Risk Assessment

Low — this only changes desktop copy for bare ACP internal errors; structured and detailed error handling is unchanged.

References

• Complements fix(acp): preserve JSON-RPC error data #2436, which preserves JSON-RPC error data when adapters provide it
• Validation: desktop checks; full desktop test suite (3,819/3,819); focused friendlyAgentLastError tests (32/32); desktop typecheck
• Repository-wide just ci reaches an unrelated existing macOS clippy failure for Linux-only helpers in desktop/src-tauri/src/linux_media.rs

Generated with Codex

[Chessing234]

thanks. please call out any migration/restart step in the pr body so operators dont miss it