self-development: Axon tasks other than axon-worker should not open a PR

[gjkim42]

If other axon-workers open a PR, I cannot give feedback, since it may not have an issue or it does not have the consistent branch name.

Prevent other axon tasks from opening the PRs.

[axon-agent]

🤖 Axon Agent @gjkim42

PR #445 is ready for review.

Changes:

• Inject AXON_TASKSPAWNER env var into agent containers from the axon.io/taskspawner label, so agents know which TaskSpawner created them
• Remove PR creation instructions from axon-fake-user and axon-fake-strategist prompt templates, replacing them with explicit "Do NOT create PRs" directives
• axon-triage already had the correct restriction

All tests and verification pass.