fix(client,spec): one owner for the README install-example pin, and declare the union's dropped refinements

`main` landed #18771, which fixes the same README example and adds
`readme-package-install-example.test.ts` — a corpus reader over every
`packages.install` literal in the published README. Two files reading the same
README to ask the same question is a duplicate owner, so the F4 additions move
INTO that file and the branch's own copy is dropped: the corpus is now parsed
against `PackageInstallBodySchema` wrapped exactly as `packages.install` sends
it and bare as the door reads it, and the pre-#18058 literal is pinned refused
so a schema relaxation cannot make the block vacuous. The README keeps main's
landed literal (`type: 'plugin'`, no deprecated `namespace`) plus this card's
prose and the `overwrite` opt-in line.

`api/PackageInstallBody` is a new published schema whose bare branch carries the
`navigationContributions` refinements `z.toJSONSchema()` cannot project, so
`dropped-refinements.baseline.json` — which landed on main after this branch
forked — gains the entry `build-schemas.ts` printed, beside the identical
`api/PackageInstallRequest` and `api/InstallPackageRequest` rows.

Claude-Session: https://claude.ai/code/session_01LvwGppdonww4zGLWZo5rho
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

packages/spec/dropped-refinements.baseline.json

@@ -402,6 +402,12 @@
         "data.titleFormat.options[1]"
       ]
     },
+    "api/PackageInstallBody": {
+      "sites": [
+        "options[1].navigationContributions.element.items.element.lazy.options[0]",
+        "options[1].navigationContributions.element.items.element.lazy.options[0].visible.options[1]"
+      ]
+    },
     "api/PackageInstallRequest": {
       "sites": [
         "manifest.navigationContributions.element.items.element.lazy.options[0]",