feat(health): canonical artifact registry and W019 unrecognized-file lint (#2448) (#2488)

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

docs/INVENTORY-MANIFEST.json

@@ -253,6 +253,7 @@
       "workstream-flag.md"
     ],
     "cli_modules": [
+      "artifacts.cjs",
       "audit.cjs",
       "commands.cjs",
       "config-schema.cjs",