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/0095_retrieval_log_suppressed_count.py

@@ -1,4 +1,4 @@
-"""add retrieval_logs.suppressed_count — tell a ranker decline from a repeat (#3497)
+"""add retrieval_logs.suppressed_count — tell a ranker decline from a repeat
 
 Revision ID: 0095
 Revises: 0094
@@ -20,9 +20,8 @@ the dropped rows never come back and there is nothing to count.
 NULLABLE, AND THE NULL IS THE POINT. A surface that does not measure
 suppression stores NULL, not 0, and the readout renders it as "not measured"
 rather than "none". Defaulting to 0 would make an unmeasured surface look like
-a perfectly clean one — the exact substitution of an artifact for a
-measurement that #3311 made and that #3497 exists to correct. Doing it again,
-in the migration that fixes it, would be its own small joke.
+a perfectly clean one — the substitution of an artifact for a measurement
+that this column exists to prevent.
 
 No backfill for the same reason: existing rows genuinely do not know, and
 saying so is the honest state. `retrieval_logs` is not restored from backup,