Close plan 98: archetypes replacement is complete (#222)

All acceptance criteria verified — internal/archetypes removed,
docs/background/archetypes gone, generated-section moved to
docs/background/concepts/, archetypes command exits 2, tests pass.

https://claude.ai/code/session_01DK5oDMP8jmsuiE76kbycZE

Co-authored-by: Claude <noreply@anthropic.com>

Author: jeduden

PLAN.md

@@ -58,5 +58,5 @@ footer: |
 | 95  | ✅     | opus   | [Kind/rule resolution observability via `kinds` subcommand](plan/95_kind-rule-resolution-cli.md)     |
 | 96  | ✅     | sonnet | [Adopt kinds in mdsmith repo and ship the docs](plan/96_kinds-adoption-and-docs.md)                  |
 | 97  | ✅     | opus   | [Deep-merge for kinds and overrides](plan/97_deep-merge-config.md)                                   |
-| 98  | 🔲     | sonnet | [Replace `archetypes` with `kinds`](plan/98_replace-archetypes-with-kinds.md)                        |
+| 98  | ✅     | sonnet | [Replace `archetypes` with `kinds`](plan/98_replace-archetypes-with-kinds.md)                        |
 <?/catalog?>

plan/98_replace-archetypes-with-kinds.md

@@ -1,7 +1,7 @@
 ---
 id: 98
 title: Replace `archetypes` with `kinds`
-status: "🔲"
+status: "✅"
 model: sonnet
 summary: >-
   Remove the `archetypes` CLI subcommand, config key,