docs: add APM ecosystem section to README #97
Description
Summary
Add a "Works with APM" section to the README explaining how agentrc-generated instructions integrate with APM for distribution, sharing, and governance across teams.
Why
Developers finding agentrc should discover APM as the natural next step for sharing their generated instructions. Developers finding APM should discover agentrc as the best way to author package content. Cross-documentation makes the ecosystem discoverable.
What
A short README section (Generate → Share → Govern) placed after "What gets generated", noting that .instructions.md files are directly compatible with APM.
Companion PR in APM repo: microsoft/apm#509
Part of: #96