Two always-loaded agent-instruction surfaces (AGENTS.md and .github/copilot-instructions.md) with no precedence rule

Summary

This repo carries two always-loaded agent-instruction surfaces covering the same ground: AGENTS.md (with CLAUDE.md as a symlink to it) and .github/copilot-instructions.md. Both describe build commands, architecture, and conventions. Nothing states which one wins, so they drift independently - and they already have.

The drift is not hypothetical

The CI Node matrix is currently stated in three places, and two of the three are wrong:

An agent or contributor reading either prose copy gets a wrong answer about what CI actually runs, and the README copy is wrong by two major versions.

layer5io/sistent#1780 fixes those two prose copies by reducing them to a pointer at the workflow, which is the pattern AGENTS.md already prescribes for itself ("Do not repeat what the codebase already shows; point to the authoritative file or command instead"). That closes the immediate factual error but not the structural one.

The structural question

AGENTS.md has a ## Maintaining this file section setting a deliberate bar: keep it to knowledge useful to almost every session, point at authoritative sources rather than restating them, prefer rewriting over appending. .github/copilot-instructions.md has no equivalent governance and no stated relationship to AGENTS.md, so:

• a maintainer updating one has no signal that the other exists;
• an agent loading both gets the same territory twice, with no precedence rule for conflicts;
• every fact restated in both is a future drift like the one above.

What to decide

One of:

1. Retire copilot-instructions.md, or reduce it to a short pointer at AGENTS.md. Copilot reads whatever is at that path; a stub that redirects costs nothing and removes the second copy.
2. Keep it, and give it a scope that does not overlap - Copilot-specific mechanics only, with everything shared living in AGENTS.md.
3. Keep both as-is and accept the duplication, in which case say so explicitly in both files so the next maintainer knows the redundancy is intentional and updates both.

Whichever is chosen, the outcome should be written down in AGENTS.md's ## Maintaining this file section, since that is the file that currently claims to govern agent knowledge in this repo.

Raised from the schema-consumer audit in layer5io/sistent#1780, which found the Node-matrix discrepancy while checking whether the change had invalidated any documented fact. Deliberately not resolved there - fixing the wrong facts is in scope for that PR; deciding whether the second instruction surface should exist is not.

Read AGENTS.md, especially its Maintaining this file section, then compare it with .github/copilot-instructions.md and the documented CI references in .github/workflows/node-checks.yml and README.md. Decide and document the intended relationship between the instruction files, ensuring the chosen policy and any required pointers or scope boundaries are written in AGENTS.md and reflected in the affected files.