fix: strip grouped headings from the four drifted fragments

Ceremony is a flat repo (#112 D3); changelog.d/135.md, 137.md, 144.md and
151.md landed with a '### Fixed'/'### Changed' heading, each individually
legal to changelog_fragment_problem, and together they made the directory
mixed-shape — 'bin/changelog-assemble 0.3.0 --check' refused on main.
Delete the heading line and its following blank line from each; every
bullet stays byte-identical (#157 D1). No fragment for this PR: its whole
diff is unpublished fragment text (#157 D2).

Closes #158

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

changelog.d/135.md

@@ -1,5 +1,3 @@
-### Fixed
-
 - `drills/README.md` no longer tells the builder to delete the scratch repo —
   a step no fleet identity can perform, because `delete_repo` is deliberately
   absent from bot tokens. The builder's end state is **archive**

changelog.d/137.md

@@ -1,5 +1,3 @@
-### Fixed
-
 - `blocker:unrequested` now clears the moment the panel is asked: the labels
   caller (and the `docs/CONSUMERS.md` stub) listens on `review_requested` and
   `review_request_removed`, so the one event that falsifies the label — or

changelog.d/144.md

@@ -1,5 +1,3 @@
-### Fixed
-
 - The `docs/CONSUMERS.md` labels-caller stub lists the same `issues:` types
   as ceremony's own caller — `edited` and `reopened` included — so a consumer
   adopting the stub wakes when an issue body's `Blocked by #N` declaration is

changelog.d/151.md

@@ -1,5 +1,3 @@
-### Changed
-
 - Doctrine names the second `Closes #N` exception: a same-repo PR whose
   authorizing issue marks an acceptance criterion post-merge uses `Refs #N`,
   and triage closes the issue by hand on the evidence — merging #143