Plan 99: generalize to capture directive with user-extensible types

Agent-Logs-Url: https://github.com/jeduden/mdsmith/sessions/97edc98b-35e8-4530-a321-3f2a1d6bbd40

Co-authored-by: jeduden <1117699+jeduden@users.noreply.github.com>

Author: Copilot

PLAN.md

@@ -34,5 +34,5 @@ footer: |
 | 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)                        |
-| 99  | 🔲     |        | [Screenshot directive and capture command](plan/99_screenshot-directive.md)                          |
+| 99  | 🔲     |        | [Capture directive and extensible builder](plan/99_screenshot-directive.md)                          |
 <?/catalog?>