Suppress A2A worker channel follow-ups

[pmbstyle]

Summary

• mark inbound A2A peer tasks as channel-followup suppressed
• skip user/channel worker-result and instruction follow-ups for suppressed correlations
• keep system worker result memory intact while avoiding Telegram delivery attempts for A2A-only conversations

Validation

• uv run black src\octopal\runtime\octo\core.py src\octopal\runtime\octo\turn_state.py src\octopal\runtime\octo\followup_pipeline.py src\octopal\interop\a2a\routes.py tests\test_a2a_gateway.py tests\test_heartbeat.py
• uv run ruff check src\octopal\runtime\octo\core.py src\octopal\runtime\octo\turn_state.py src\octopal\runtime\octo\followup_pipeline.py src\octopal\interop\a2a\routes.py tests\test_a2a_gateway.py tests\test_heartbeat.py
• uv run pytest tests\test_a2a_gateway.py tests\test_heartbeat.py -q
• local A2A QA instance: qa_worker_micro_report passed and logged Internal worker follow-up skipped with reason a2a_peer_message