docs(6210): acceptance sweep — src, scripts and frontend leftovers described in the present tense

The acceptance greps found what the per-area passes missed:

- src/scribe: "Phase 7/8" removal notes in projects, settings and
  user_profile; a paragraph promising an auto-summary side effect that no
  longer exists; user_profile's claim that "upcoming MCP profile tools" read
  the profile, which nothing does; an instance count and instance rule
  titles in plugin_context; "MILESTONE 323 DECIDED" in rulebooks.
- scripts/: check_plugin's header and inline comments were a history of
  the incidents behind each guard, with issue numbers in two failure
  messages; check_dangling_styles named this app's classes as its examples;
  check_design_tokens, measure_duplication and mint_plugin_version cited
  issues and checklists.
- frontend/src: forty-odd comments with four-digit issue numbers that the
  earlier frontend scan filtered out as hex colours, plus "used to" and
  "Phase N" notes.

Kept: illustrative ids (`#4181` as a bare-id example, `PR #198`), migration
Create Date headers and one migration's dated data floor, and the plugin
skills' worked examples of how to cite a record.

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

scripts/measure_duplication.py

@@ -7,8 +7,7 @@ A DRY audit made of several passes ends with a question no single pass can
 answer: did the audit as a whole reduce duplication, and did it create any? The
 second half is the one that matters. A pass that shortens two statements can
 leave them byte-identical to a third, and the new copy is invisible to that
-pass's own scan because it did not exist when the scan ran (#4745: a retro over
-a 17-pass audit found exactly this, and folded it into a view). This script is
+pass's own scan because it did not exist when the scan ran. This script is
 the close-out measure the DRY Pass process names, so the measure is a tool and
 not something each audit improvises.