docs: discover shared organism standards

[Jammy2211]

Summary

Add the canonical shared-standards discovery block generated by PyAutoMind's repo-sync tool. Agents can discover the relevant Brain contract from an isolated checkout; board-specific guidance appears only on declared board owners. Existing repository instructions are preserved byte-for-byte.

Part of PyAutoLabs/PyAutoMind#474. Canonical source: shared standards policy. Human /prm; no merge is requested by this PR creation.

Scripts Changed

None. Only AGENTS.md changes; no library API, scientific behavior, notebooks, release workflows or data changes.

Validation

• Target-scoped generated-block drift check passed for PyAutoReduce.
• Byte audit: all pre-existing AGENTS content preserved, exactly one generated block added.
• Generator: 652 Mind tests pass, including 33 standards tests and adversarial scoped-write/CRLF cases.
• git diff --check passed. Required repository CI remains authoritative; full scientific suites were not rerun locally for an instruction-only addition.
• Heart YELLOW reason set is unchanged and explicitly acknowledged in-session (three aged PRs and stale Nerves rehearsal evidence).