docs(6207): the remaining migrations state their rationale in terms of the data

Record, milestone, phase and plan citations go from every migration header;
the "end before start" fix (0043) and the suppressed-count column (0095)
state the condition they prevent rather than the incident. 0053 and 0055
pointed at design specs under docs/superpowers that are not in the repo;
those pointers are removed.

Kept: alembic's generated "Create Date:" headers (template metadata, not
rationale), 0121's ARM_BORN justification beside the constant, and 0051's
illustrative "update task 42" format example.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

alembic/versions/0076_note_supersessions.py

@@ -4,7 +4,7 @@ Revision ID: 0076
 Revises: 0075
 Create Date: 2026-08-07
 
-Step 1 of milestone #278. Structure only — nothing reads or writes the new
+Structure only — nothing reads or writes the new
 table yet, and nothing behaves differently after this runs.
 
 ## What the table is for
@@ -39,7 +39,7 @@ and not for notes, so the column looked like the notes-side half of that
 feature, modelled and abandoned.
 
 It is dropped rather than repurposed for supersession, and the distinction is
-the point (#2483): consolidation folds several records into one survivor and
+the point: consolidation folds several records into one survivor and
 destroys the originals. Merging two snippets is lossless — one helper, several
 call sites. Folding two dev-logs means writing a summary and losing what each
 actually said. Supersession is the opposite act: both records survive, and the