fix(pipeline): keep the plan number in a completed plan's filename

plugins/pipeline/skills/complete-plan/SKILL.md

@@ -17,10 +17,39 @@ Steps:
 3. **Verify the plan is honest.** Skim the plan's acceptance criteria against what was actually implemented. If implementation diverged, ask the user to update the plan file first, then re-run this skill.
 4. Compute the destination filename:
    - `YYMMDD` = today's date in UTC (`date -u +%y%m%d`).
-   - `NNNN` = next zero-padded daily index: scan `plans/completed/$YYMMDD.*.md`, take highest existing + 1, else `0001`.
+   - `NNNN` = **the plan's own number**, zero-padded to four digits — `007-foo.md` → `0007`. A plan whose source filename carries no number takes `0000`.
    - `slug` = the slug portion of the source filename (after the `NNN-` prefix).
    - Destination: `plans/completed/$YYMMDD.$NNNN.$slug.md`.
+   - **If that path already exists, stop and ask.** Two plans cannot share a number, so a collision means the source was already completed under another name, or a number was reused.
 5. Run `git mv <source> <destination>` so history is preserved.
-6. Report the move and the new path.
+6. **Re-point the citations.** A plan is cited by number — from other plans, from a `plans/README.md` index, from decision records, from commit bodies. Grep the repository for the number and update anything that describes it as active. If the project has a check for dangling plan references, run it.
+7. Report the move and the new path.
 
-Do not refactor or alter the plan's contents during the move.
+Do not refactor or alter the plan's contents during the move. Anything the plan
+leaves behind — deferred work, findings refuted rather than fixed, decisions
+owed to the owner — is written into the plan **before** this skill runs, as the
+last act of executing it.
+
+## Why `NNNN` is the plan number
+
+It was a per-day counter until 2026-08-30, and repositories completed under that
+rule hold filenames whose `NNNN` is a small ordinal unrelated to the plan.
+**Leave those alone** — renaming history to match a new rule breaks the links
+that already point at it, and the rule earns its keep on the plans still to
+come.
+
+The counter was replaced because it lost the only identifier anyone cites. After
+the move, nothing in the filename said which plan the file was, so every "plan
+013" reference in a README, a decision record or another plan's dependency graph
+became unresolvable — and a project that checks for dangling plan references
+fails the moment a plan completes. The counter also failed at being an ordinal:
+across the projects using it, seven pairs of files share a `YYMMDD.NNNN` prefix,
+because step 4's scan is easy to skip and nothing downstream notices.
+
+A plan number is unique per repository by construction, so it needs no scan, and
+it is the string a reader already has in hand.
+
+**What would reverse this:** a project that numbers plans non-uniquely, or one
+that reuses numbers across a `plans/history/` archive. Neither exists today; the
+first one to appear is the argument for going back to a counter, and it would
+need the collision guard in step 4 either way.