docs(adr): move ADR 007's amendment to its own file, stating only what it changes (improvise-cq1q.1)

On the user's instruction an ADR states only what its decision changes,
and mentions an existing convention only where it conflicts. The accepted
ADR 007 is restored to main's text plus a one-line pointer, so merging
this PR keeps it; the amendment is context/adr/007-stable-identities-
amendment-1.md (280 lines, from 784), with a table of the accepted rules
it replaces. The doc site lists the new file, and core.md section 20
points at it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/code/core.md

@@ -1714,7 +1714,8 @@ underscore-collapsing step exists. A trailing run survives, so `East ` slugs
 to `east_`. Only *leading* underscores are removed (step 4).
 
 **ADR 007's proposed amendment changes these rules, and this code predates it.**
-The amendment (PR #326) is not yet approved. Its clause 4:
+The amendment (`context/adr/007-stable-identities-amendment-1.md`, PR #326) is not
+yet approved. Its clause 4:
 - every character outside `a`–`z`, `0`–`9` and `-` becomes `_` instead of being
   stripped (not only U+0020), so `Region(code)` slugs to `region_code`, while
   `Q1-2024` keeps its `-`;
@@ -1727,7 +1728,7 @@ The amendment (PR #326) is not yet approved. Its clause 4:
 This section, `slugify`, and the tests
 `slugify_collapses_a_run_of_spaces_to_one_underscore` (its `east_` case) and
 `slugify_keeps_an_underscore_the_display_already_had` describe the earlier rule
-above. They change in a follow-up once the amendment is approved. The ADR's
+above. They change in a follow-up once the amendment is approved. The amendment's
 clause 4 lists every difference.
 
 **The allocator records every id it admits and releases none.** That is the