RFR: 8527: Add AGENTS.md file

[Marcus Hirt]

First basic AGENTS.md file for the JMC project.

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Commit messages:
 - 8527: Add AGENTS.md file

Changes: https://git.openjdk.org/jmc/pull/704/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=704&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8527
  Stats: 142 lines in 1 file changed: 142 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jmc/pull/704.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/704/head:pull/704

PR: https://git.openjdk.org/jmc/pull/704