docs(6208): the remaining frontend comments drop milestone, step and rule citations

Components, stores, api clients, router, types and shared CSS now state what
each piece is and why, without the milestone that introduced it. diff.ts
names its consumers instead of the copy history; stores/notes and tasks state
their current scope instead of the cleanup that set it; theme.css states the
token practice instead of the sweep that established it.

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

frontend/src/assets/rules-shared.css

@@ -20,8 +20,8 @@
    milestone (tier, then verification) and were byte-identical; a third would
    have drifted. The pane's italic serif title is inherited by anything inside
    it, so the chip resets family and style explicitly. */
-/* A rule with no trigger cannot be retrieved, and since milestone 394
-   retrieval is the only delivery — so this marks a rule that will never
+/* A rule with no trigger cannot be retrieved, and retrieval is the only
+   delivery — so this marks a rule that will never
    reach a session. Warning rather than error: the rule is not broken, it is
    unreachable, and the fix is one field away. */
 .rule-chip-inert {