fix(remote): route code agent approvals

[DylenTurnbull]

Summary

• route exact top-level mj-code-agent native approval forms to remote approval cards
• support explore_agent, explore_agents, code_agent, code_agent_continue, and code_agent_cancel
• keep nested Eitri elicitations declined and reject malformed or lookalike forms

Validation

• cargo test
• cargo fmt --check
• git diff --check
• manual remote approval-card validation for allow, decline, explore, continue, and cancel

Closes #420