docs: verification-pass fixes (af server, dev_mode hallucination, Makefile PYTHONPATH)

[AbirAbbas]

Summary

Third pass focused on verifying that everything I documented actually works. Three real hallucinations from the prior PRs (#27 / #28), all caught by tracing each documented claim back to the code:

• The Troubleshooting note "With dev_mode=True (the default), PR-AF retries registration on a 30s loop" was wrong on both counts. PR-AF's app.py constructs Agent() without dev_mode, so the agentfield SDK default of False applies, and the agentfield SDK has no automatic registration retry loop — failed first-time registration just logs a warning. Reworded to tell the user to restart the service after the control plane is up.

• The from-source command af (Quick Start) and CONTRIBUTING.md install instructions implied the AgentField CLI ships with the Python agentfield package via pip install -e ".[dev]" and could be invoked as python -m agentfield server. Neither is true: the af CLI is a Go binary installed via curl -fsSL https://agentfield.ai/install.sh | bash and the command is af server, not af. Fix the commands in both files and add the install one-liner up front.

• make test invoked pytest tests/ with no PYTHONPATH, so tests importing pr_af.* failed with ModuleNotFoundError unless the user had also done pip install -e . first. Set PYTHONPATH=src in the Makefile's test target so tests run cleanly without an editable install. While here, document the pre-existing test_cost_tracker::test_async_log_success failure in CONTRIBUTING.md so contributors know it's not their patch.

Test plan

• CI green
• Run `make test` locally and confirm it surfaces the real `test_async_log_success` failure (i.e. PYTHONPATH=src worked) instead of the prior `ModuleNotFoundError`
• Run `make lint` and confirm `All checks passed!`

🤖 Generated with Claude Code