Clarify that project-specific rules/API-contracts belong in AGENTS.md, not .github/copilot-instructions.md

[ptr727]

Context

Surfaced while re-syncing a derived repo (ptr727/LanguageTags) against the template (downstream heads-up issue ptr727/LanguageTags#176).

Gap

The verbatim-carry guidance in AGENTS.md ("Files and Sections Derived Repos Must Carry Verbatim") describes .github/copilot-instructions.md as a narrow drop-in ("the whole file is a drop-in ... drop language-specific style pointers that don't apply"), but it does not say where project-specific conventions should live instead.

A derived repo had accumulated its API-design rules and public-API contract notes (factory-only construction, immutability, parse/validate/normalize semantics, normalization casing, grandfathered-tag handling) inside .github/copilot-instructions.md. That file is read by GitHub Copilot / VS Code generators specifically. When a different agent (Claude Code, Codex, Cursor, ...) reviews a PR it reads AGENTS.md and never sees those rules - so provider choice silently changes which project rules an agent honors.

Suggested clarification

State explicitly in the template that:

• Project-specific conventions and API/behavioral contracts belong in AGENTS.md (the agent-agnostic canonical file every coding agent reads).
• .github/copilot-instructions.md stays narrow - provider-specific mechanics (the Copilot review runbook) plus the inline commit/PR-title summary - and is not a home for project rules.
• Rationale: non-Copilot agents only read AGENTS.md, so any rule a reviewer must follow has to live there to be provider-independent.

This keeps the "copilot-instructions.md is narrow" contract meaningful by answering the natural follow-up ("then where do my project rules go?").

Resolution in the derived repo

LanguageTags moved its API rules into AGENTS.md ("Library API Conventions") and slimmed .github/copilot-instructions.md back to the narrow template shape.

[ptr727]

Resolved by #174 (merged to develop; this issue closes once it reaches main).

What changed

Two verbatim-carry files now answer the follow-up "then where do my project rules go?":

• AGENTS.md — the intro and the "Files and Sections Derived Repos Must Carry Verbatim" entry state that project-specific conventions and API/behavioral contracts belong in AGENTS.md (the agent-agnostic file every coding agent is directed to read), and that .github/copilot-instructions.md stays narrow: the Copilot review runbook plus the inline commit/PR-title summary.
• .github/copilot-instructions.md — the narrow-file note now points project rules at AGENTS.md, with the rationale that non-Copilot agents (Claude Code, Codex, Cursor, …) are not directed to this file and don't read it by default, so any rule a reviewer must honor has to live in AGENTS.md to be provider-independent.

(#174 also hardened the Copilot review runbook for the case where Copilot reviews but produces no inline comments — a completed clean pass, not a missing review — but that's incidental to this issue.)

Re-sync ("re-merge") instructions for downstream repos

Both files are verbatim-carry artifacts, so re-pull them once #174 lands on main:

1. Re-sync AGENTS.md and .github/copilot-instructions.md from the template main, adapting only the <owner> / <repo> / <N> placeholders in the runbook's API snippets.
2. Move any project-specific conventions or public-API contracts currently living in .github/copilot-instructions.md into a project section of AGENTS.md (e.g. a "Library API Conventions" section), leaving copilot-instructions.md narrow.
3. Re-verify line endings (CRLF/LF per .editorconfig) on the touched files and run markdownlint.

ptr727/LanguageTags already made this move (the resolution noted above); this change just makes the template state the rule so the next derived repo gets it for free.