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/0100_drop_always_on_tier.py

@@ -4,14 +4,14 @@ Revision ID: 0100
 Revises: 0099
 Create Date: 2026-09-11
 
-Milestone 394. Every rule now reaches a session by retrieval — because
+Every rule reaches a session by retrieval — because
 something it is about to do made the rule relevant — and the machinery that
 delivered rules unconditionally goes with it.
 
 WHAT GOES, AND WHERE IT CAME FROM
 
   - ``rules.tier`` and its ``ck_rules_tier`` CHECK (migration 0088). Dropping
-    the column takes the constraint with it. Rule 36 is about ADDING a value to
+    the column takes the constraint with it. DROP + ADD is about ADDING a value to
     a live whitelist, which needs DROP + ADD in the same migration; it does not
     speak to removing the column outright, and saying so here is cheaper than
     the next reader wondering whether it was forgotten.