Add root AGENTS.md guidance

[akhilles]

• add a root for coding agents
• keep the guidance concise and focused on stable repo-specific instructions
• document the final pre-commit verification step, including

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Note

Low Risk
Low risk: documentation-only change that adds repository guidance for coding agents and removes an old Cursor-specific rule file; no runtime code or behavior is affected.

Overview
Adds a new root AGENTS.md with repo-wide guidance for coding agents, including common pcb CLI commands, a repository map, documentation update expectations (notably keeping docs/pages/spec.mdx in sync with language changes), and a suggested verification checklist.

Removes the previous .cursor/rules/spec.md rule, effectively consolidating that spec-update requirement into the new root agent guidance.

^{Written by Cursor Bugbot for commit e1aace4. This will update automatically on new commits. Configure here.}

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.