Clarify PR defaults in AGENTS.md

[cristianrgreco]

Summary

• add an instruction-precedence section to AGENTS.md
• clarify that PR titles must not include automated prefixes such as [codex]
• clarify that PRs should be opened ready for review by default and only use draft when explicitly requested or intentionally not ready

Why

The current AGENTS.md guidance did not explicitly state that repository-specific instructions override generic agent defaults, and it did not explicitly define the default PR review state. Making those rules explicit reduces avoidable process mistakes in future agent-driven changes.

Impact

This is a documentation-only clarification. It does not change runtime behavior or public APIs.

Verification

• npm run format
• npm run lint

Test results summary

• npm run format: passed
• npm run lint: passed

Evidence this is not breaking

• documentation-only change with no code, package, or API changes

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