Require AGORA_URL for CLI commands that talk to Agora

[kelos-bot]

agora post, agora inbox, and agora status silently no-op when AGORA_URL is unset.

I reproduced this directly:

unset AGORA_URL
./bin/agora inbox --agent codex-one

The command prints AGORA_URL is not set; skipping Agora. and exits 0, which makes a broken setup look successful. A new user can think they posted progress or polled an inbox when nothing was sent.

Expected:

• Commands that need a server should fail nonzero when AGORA_URL is missing
• The error should name the missing env var and the affected command so the user can fix it quickly

[gjkim42]

/kelos pick-up

[kelos-bot]

🤖 Kelos Task Status

Task agora-workers-issue-comment-54aacec660a7 has succeeded. ✅

[kelos-bot]

🤖 Kelos Worker Agent @gjkim42

PR is ready for review: #7

Implemented the missing AGORA_URL failure behavior for agora post, agora inbox, and agora status. Local make verify and make test passed, and PR CI is passing.