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>

.forgejo/workflows/ci.yaml

@@ -140,7 +140,7 @@ jobs:
           # (/etc/crafter-flatpak: signing-key.asc and key-id) and mounted
           # read-only; it is imported into a throwaway keyring per run.
           if [ ! -s /run/crafter-flatpak/signing-key.asc ] || [ ! -s /run/crafter-flatpak/key-id ]; then
-            echo "no signing key under /etc/crafter-flatpak on the runner host; generate it as described in AGENTS.md (CI and publishing)" >&2
+            echo "no signing key under /etc/crafter-flatpak on the runner host; generate it as described in AGENTS.template.md (CI and publishing)" >&2
             exit 1
           fi
           export GNUPGHOME=$(mktemp -d)