One canonical agent file, every vendor path a thin import

## Spark

AGENTS.md as the single source, with CLAUDE.md and the other vendor paths reduced to thin imports, got filed across seven repos on one day after genshin.dungeon.studio proved it. zfs-replicate went further and deleted its Copilot instruction files outright.

## Why it could be interesting

Every AI coding tool wants its own instruction file at its own path, and keeping N copies in sync fails silently — the agent reads the stale one and nothing errors. The claim: converge on one file, make the vendor paths pointers, and delete the pointer entirely when no tool at that path actually reads it.

## Open questions

- [ ] Which tools genuinely follow an import or pointer, and which need real content at their path? Needs per-tool verification, not assumption.
- [ ] Does a canonical file grow past what any single tool should load every turn? zfs-replicate#477 trimmed CLAUDE.md for exactly that.
- [ ] Is AGENTS.md the right canonical name, or is that betting on one ecosystem's convention?

## Source material

- https://github.com/dungeon-studio/genshin.dungeon.studio/pull/1417, closing https://github.com/dungeon-studio/genshin.dungeon.studio/issues/877 — the proof
- The sweep: https://github.com/alunduil/blog.alunduil.com/issues/587, https://github.com/alunduil/alunduil-chezmoi/issues/635, https://github.com/alunduil/alunduil-infrastructure/issues/427, https://github.com/alunduil/collection-json.hs/issues/299, https://github.com/alunduil/siren-json.hs/issues/154, https://github.com/alunduil/woodland-generators/issues/524, https://github.com/dungeon-studio/dungeon-studio-infrastructure/issues/35
- Deletion instead of a pointer: https://github.com/alunduil/zfs-replicate/issues/484 (closed) and https://github.com/alunduil/zfs-replicate/pull/667
- Token cost: https://github.com/alunduil/zfs-replicate/issues/477 (closed) and https://github.com/alunduil/zfs-replicate/pull/661