* fix(artifacts): register RETROSPECTIVE.md as canonical planning artifact
Adds RETROSPECTIVE.md to CANONICAL_EXACT in artifacts.cjs so gsd-health
no longer raises W019 after any /gsd-complete-milestone run. The file was
established as a living artifact in PR #644 but omitted from the W019
registry created in PR #2488.
Closes#3198
* chore(changeset): point pr metadata to #3200
Adds artifacts.cjs with canonical .planning/ root file names, W019 warning
in gsd-health that flags unrecognized .md files at the .planning/ root, and
templates/README.md as the authoritative artifact index for agents and humans.
Closes#2448