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/0107_system_embeddings.py

@@ -1,4 +1,4 @@
-"""system_embeddings — a charter becomes an ANSWER, not just a filter (#4251)
+"""system_embeddings — a charter becomes an ANSWER, not just a filter
 
 Revision ID: 0107
 Revises: 0106
@@ -13,8 +13,7 @@ to one, and an agent asking where a record belonged had to read every charter
 or guess.
 
 The fourth sibling of note_embeddings (0067), rule_embeddings (0089) and
-milestone_embeddings (0102), and for the same reason note 3163 gives about the
-third: the row could be shared, the search cannot. "Where does this belong?"
+milestone_embeddings (0102), and for the same reason as the third: the row could be shared, the search cannot. "Where does this belong?"
 is a different question from "what prior art is there?", and no note or rule
 search can answer it, because a charter is not a note.