feat(diagnostics): expose opt-in handler failure messages

[jmgilman]

Summary

Closes #58.

• Add codemode.AgentError for handler-authored, explicitly agent-facing failures. Find wrapped instances with errors.As; never disclose surrounding errors.
• Sanitize only Message: replace non-printable runes with spaces, repair invalid UTF-8, and truncate on a rune boundary to 256 bytes including ....
• Carry approved detail over the existing parent-owned native_abort frame. Preserve the terminal abort protocol and prevent child-origin capability-failure detail.
• Preserve coarse root Error() and errors.Is behavior; format capability failed: <detail> in MCP.
• Document the API, sanitization, and host responsibility not to disclose secrets.

Verification

• Ran a standalone MCP client/server smoke program through a real re-executed worker: direct and wrapped AgentError, ordinary handler failure, and oversized control-filled message. Removed the smoke program afterward.
• Added MCP acceptance coverage for Unicode boundaries, invalid UTF-8, empty/typed-nil errors, and ordinary/panic/policy/conversion nondisclosure. The existing Echo model-derived diagnostics through the coarse error taxonomy #24 MCP nondisclosure test is unchanged.
• Verified native abort detail reaches Starlark's evaluator error text and does not authorize a child terminal capability-failure frame.
• Repository validation uses mise exec -- moon run root:check --summary minimal (format, lint, build, MCP smoke, race suite, docs build).