Add a status checklist as the authoritative plan-progress record

Completion status previously lived only in my in-session TodoWrite list
(ephemeral, doesn't survive the session) or had to be inferred from which
steps had an entry in the execution log. Neither is a durable, explicit
answer to "what's left."

Adds a markdown checklist at the top of each plans-executions file, one
line per plan step, checked in the same commit as that step's entry. A
new session resuming this plan can read the file alone and know exactly
where to pick up.

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

CLAUDE.md

@@ -104,6 +104,15 @@ originating plan in `docs/plans/`, so the two files pair up 1:1. Create the file
 section per step after that, in chronological order (bottom of the file), not
 newest-first like `CHANGELOG.md`.
 
+**Status checklist.** The top of the file, above the first step's entry, is a markdown
+checklist with one line per step from the plan (`- [ ] Step 0 — <title>`), so a single
+glance shows what's done and what's left without reading prose or cross-referencing
+`git log`. Check a box in the same commit that adds that step's entry — never leave a
+completed step unchecked or a not-yet-done step checked. This checklist is the
+authoritative status record; an in-session todo list (e.g. `TodoWrite`) is ephemeral
+and does not substitute for it — a new session resuming this plan should be able to
+read this file alone and know exactly where to pick up.
+
 Keep the prose light — a few sentences on what was done, plus anything genuinely
 interesting or non-obvious about how it went (a judgment call made, a spec gap found,
 something that didn't work as expected). This is not a duplicate of the plan or the