docs: keep this repository's agent notes in AGENTS.template.md

This repository now uses the template itself. AGENTS.md is written by
the self-host build and `test` and is gitignored, so the notes
Crafter.Math ships reach the agents working here, and a change to them
no longer leaves a tracked file modified, which also made every build
afterwards a dirty-tree version, a new crafter.build-lib variant and a
full rebuild.

The gotcha that described the old arrangement now says where to edit,
that a fresh clone or worktree has no AGENTS.md until its first build
(./build.sh does not write it), and what an older harness does to the
generated file. The paths table and the never-commit list name both
files. The CI error for a missing signing key and a manifest comment
point at the template, since the forge no longer shows an AGENTS.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -5,3 +5,5 @@ share/crafter-build/
 .claude
 # written by CI next to the manifest so relative source paths still resolve
 packaging/flatpak/ci-manifest.yaml
+# written by every build and test run from AGENTS.template.md
+/AGENTS.md