fix(#3224): register WINDOWS.md as a canonical .planning/ artifact (#3369)

* test(#3224): assert WINDOWS.md is a canonical .planning/ artifact

The broken-windows ledger (.planning/WINDOWS.md, written by gsd-core's own
windows command) was absent from CANONICAL_EXACT, so validate health flagged
it W019 'Unrecognized' with advice to delete a file that can gate /gsd-ship.
Add WINDOWS.md to the expected-canonical list + a dedicated predicate test.

RED — fails on next; fix follows.

* fix(#3224): register WINDOWS.md as a canonical .planning/ artifact

CANONICAL_EXACT (src/artifacts.cts) was never updated when the broken-windows
capability (#1950/#2441) started writing .planning/WINDOWS.md. validate health
therefore flagged the ledger W019 'Unrecognized' with fix advice to archive or
delete it — a file gsd-core itself produces (src/broken-windows.cts,
LEDGER_FILE_NAME) and that can gate /gsd-ship under workflow.windows_enforce.

Add WINDOWS.md to CANONICAL_EXACT, per the registry header's own maintenance
mandate ('Add entries here whenever a new workflow produces a .planning/ root
file'). isCanonicalPlanningFile now returns true for it, suppressing the false
W019. Existing W019 behavior for genuinely-unrecognized files is unchanged.

* chore(#3224): add changeset fragment

* chore(#3224): backfill changeset PR number (#3369)

---------

Co-authored-by: sim <sim@local>

.changeset/calm-fern-meadow.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 3369
+---
+**`gsd-tools validate health` no longer flags `.planning/WINDOWS.md` as an unrecognized file** — the broken-windows ledger that gsd-core's own `windows` command writes is now registered as a canonical `.planning/` artifact. Previously the W019 warning advised archiving or deleting a file that, with `workflow.windows_enforce` on, gates `/gsd-ship`. (#3224)