ci: track the spec's HEAD — the app conforms to the living spec

Design change, at the maintainer's direction: the gate now checks out
feedpak-spec at HEAD instead of a pinned SHA. .feedpak-spec-ref, its
40-char validation step, and the pin-bump machinery are gone.

Rationale: it is vital that the app conforms to the spec — the current
spec, not a snapshot. The pin bought determinism at the cost of a
maintenance loop (bump PRs, a PAT, weekly latency) and a window where the
gate verified against a stale spec. Tracking HEAD makes the dev flow fully
self-serve with zero upkeep: gated PR -> FEP -> spec merge -> re-run
checks -> green. Nothing to bump.

The trade-off is accepted with eyes open, and the docs state it: the
normal FEP is additive and can only loosen the gate, so it cannot redden
anyone's PR. Only a breaking spec change (rare, deliberate, MAJOR per the
spec's compatibility policy) turns PRs red repo-wide — which is the
correct org-wide signal that the app is out of conformance. The CI job
logs the spec SHA each run verified against, so any red run is
reproducible.

Failure messages now also say why it matters beyond the one PR (also at
the maintainer's direction): non-conformance that lands shows up as red CI
on every teammate's PR until it is resolved, and only its author can clear
it — the FEP route keeps everyone else unblocked. Tone softened throughout
(the exceptions-file header now explains rather than shouts).

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

.github/pull_request_template.md

@@ -1,17 +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`)
+## 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#___` (once it merges, re-run this PR's checks and the gate goes green)
+
+## Checklist
+
+- [ ] `CHANGELOG.md` `[Unreleased]` updated (user-visible changes)
+- [ ] Tests added/updated for new behaviour
+- [ ] Commits are DCO signed off (`git commit -s`)