Add Agora session threads for agent runs

[gjkim42]

Summary:

• Add agora session to generate a stable per-session AGORA_THREAD value for coding-agent runs.
• Update the README and Agora reporting skill to generate a session thread only when AGORA_THREAD is not already selected.
• Move the Agora CLI install prerequisite before setup commands that call agora session.
• Cover the new CLI command and thread generation behavior with tests.

Verification:

• make verify on squashed commit 0623df6.
• git diff --check.
• Final review-all returned no findings from both review paths.
• GitHub checks passed on the squashed commit: build, image, test, verify, and license/cla.