ci: close the escape hatches — the FEP process is the only route

The gate's purpose is to make a non-conforming change *not merge*, so the
person merging must stop and decide whether to take it through the format
process. The escape hatches defeated exactly that: a developer who did not
want to write a FEP could name their key `x-whatever`, or append an entry to
feedpak-spec-exceptions.yml with any issue link, and merge. Both were
self-serve and in-repo. That is a speed bump with a signed excuse note, not a
gate.

The relief valve is the FEP process itself, not something in this repo. The
spec's governance already says so: "A change is not part of the format until
it lands here."

Removed the `x-` prefix bypass. It was invented here, not in the spec — the
spec reserves no experimental namespace. Its "unknown keys are reserved for
forward-compatibility" rule is about *tolerating* other implementations'
keys, not a licence for core to mint its own.

feedpak-spec-exceptions.yml is now a CLOSED grandfather list. A new check
(allowlist-closed) diffs it against the base branch and fails any PR that
ADDS an entry; removal stays allowed, so the list can only shrink. Deleting
an entry does not by itself pass the gate — key-coverage still fails while
core reads the key, so the entry goes when the code goes.

Every failure message now points at the FEP process and at bumping
.feedpak-spec-ref to the merged spec SHA, which is the one supported way a
new manifest key reaches core.

CI fetches the base branch to diff the allowlist; the bootstrap flag covers
the one case with no baseline — the PR introducing the gate.

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

.github/workflows/ci.yml

@@ -139,8 +139,11 @@ jobs:
       - uses: actions/checkout@v4
         # This job runs repository code (tools/check_spec_conformance.py) and
         # never pushes; don't leave the token in git config for it.
+        # fetch-depth: 0 so the base branch is available — the gate must prove
+        # the exception allowlist didn't grow in this PR.
         with:
           persist-credentials: false
+          fetch-depth: 0
 
       - uses: actions/setup-python@v5
         with:
@@ -183,8 +186,24 @@ jobs:
           # changed neither this repo nor the spec.
           pip install 'jsonschema==4.26.0'
 
+      - name: Fetch the base branch's exception allowlist
+        id: baseline
+        run: |
+          # The allowlist is closed: it grandfathers keys that predate this gate
+          # and may only shrink. Prove that by diffing against the base branch —
+          # without this, anyone could append an entry and route around the FEP
+          # process from inside this repo.
+          git fetch --no-tags --depth=1 origin main
+          if git cat-file -e FETCH_HEAD:feedpak-spec-exceptions.yml 2>/dev/null; then
+            git show FETCH_HEAD:feedpak-spec-exceptions.yml > "$RUNNER_TEMP/baseline-exceptions.yml"
+            echo "args=--baseline-exceptions $RUNNER_TEMP/baseline-exceptions.yml" >> "$GITHUB_OUTPUT"
+          else
+            # Only true until the PR that introduces this gate lands.
+            echo "args=--bootstrap-allowlist" >> "$GITHUB_OUTPUT"
+          fi
+
       - name: Check feedpak spec conformance
-        run: python tools/check_spec_conformance.py --spec .feedpak-spec
+        run: python tools/check_spec_conformance.py --spec .feedpak-spec ${{ steps.baseline.outputs.args }}
 
   lint:
     # Maintainer/CI-only size + module-hygiene gate (constitution Principle I: