docs: make FEP-first impossible to miss before the gate fires

The CI gate catches spec drift at merge time; these two additions catch it
at write time, which is where "developer didn't read the spec first"
actually happens.

CLAUDE.md (Song Formats): a spec-is-sacrosanct paragraph next to the spec
pointer — the spec defines the format, the app implements it, any new
manifest key/file/directory lands in the spec first via the FEP process,
and the gate has no in-repo bypass. AI agents and contributors both hit
this while writing feedpak-touching code, not after CI reddens.

.github/pull_request_template.md (new — the repo had only issue templates):
a feedpak-surface section requiring either "doesn't touch pack I/O" or a
link to the landed FEP + the .feedpak-spec-ref bump, plus the standing
changelog/tests/DCO checklist.

Signed-off-by: topkoa <topkoa@gmail.com>

.github/pull_request_template.md

@@ -0,0 +1,17 @@
+## What
+
+<!-- What does this PR do, and why? Link the issue it addresses. -->
+
+## feedpak surface
+
+<!-- The feedpak spec is sacrosanct: the spec defines the format, this app implements it.
+     Delete this section ONLY if your change doesn't touch how the app reads or writes packs. -->
+
+- [ ] This PR does **not** change how the app reads/writes feedpaks (manifest keys, pack files, folder layout)
+- [ ] …or it does, and the spec change landed first via the [FEP process](https://github.com/got-feedback/feedpak-spec/blob/main/CONTRIBUTING.md) — FEP / spec PR: `got-feedback/feedpak-spec#___` — and `.feedpak-spec-ref` is bumped to the merged SHA in this PR
+
+## Checklist
+
+- [ ] `CHANGELOG.md` `[Unreleased]` updated (user-visible changes)
+- [ ] Tests added/updated for new behaviour
+- [ ] Commits are DCO signed off (`git commit -s`)