fix(aws-transform-agent-toolkit): correct a2aSupported example and UpdateAgent claim

[lzongren]

Summary

Two issues in aws-transform-agent-toolkit/steering/agent-registration.md caused users to register orchestrators that the AWS Transform webapp couldn't chat with:

1. The "Complete agentCard Example" set a2aSupported: false. Users (and LLMs) following this as a template registered chat-broken agents — the webapp lists them but won't route messages to them. Flipped to true to match every other example in the file (Minimal, Orchestrator, Python, etc.).
2. A note above claimed jobOrchestratorMetadata is "fully updatable post-registration" via UpdateAgent. The registry's UpdateAgent API actually only accepts customerConfiguredAgentDependencies, marketplaceMetadata, and deprecated — there is no path to update jobOrchestratorMetadata once registered. Replaced with an accurate warning that matches troubleshooting.md.

Why this matters

The combination is a trap: users start by copying the "Complete" example (with false), assume they can fix it later, and end up unable to recover without re-registering under a new agent name.

Test plan

• Diff is 2 lines + 1 warning rewrite — no other changes
• All other a2aSupported examples in the file remain true (verified via grep)
• Updated note matches the existing accurate guidance in troubleshooting.md line 93