docs(shell-docs): use prose "public license key" in CTA copy

Three CTA `body` strings rendered "publicLicenseKey" as plain prose,
which reads as code (camelCase identifier) where the surrounding
sentence is conversational. Switches to "public license key" — the
phrase, not the prop name. Code samples and reference docs that
reference the actual `publicLicenseKey` prop are unchanged.

Author: samjulien

showcase/shell-docs/src/content/reference/hooks/useThreads.mdx

@@ -6,7 +6,7 @@ description: "React hook for listing, managing, and syncing conversation threads
 <OpsPlatformCTA
   variant="card"
   title="useThreads needs an Enterprise Intelligence Platform project"
-  body="Sign up for a free account to get a publicLicenseKey and start syncing threads."
+  body="Sign up for a free account to get a public license key and start syncing threads."
   ctaLabel="Create a free account"
   surface="docs_reference_use_threads"
 />

showcase/shell-docs/src/content/snippets/shared/premium/headless-ui.mdx

@@ -10,7 +10,7 @@ import {
 <OpsPlatformCTA
   variant="card"
   title="Headless UI requires an Enterprise Intelligence Platform license key"
-  body="Sign up for a free account to get a publicLicenseKey and unlock the headless hooks."
+  body="Sign up for a free account to get a public license key and unlock the headless hooks."
   ctaLabel="Create a free account"
   surface="docs_premium_headless_ui"
 />

showcase/shell-docs/src/content/snippets/shared/premium/observability.mdx

@@ -7,7 +7,7 @@ icon: "lucide/Activity"
 <OpsPlatformCTA
   variant="card"
   title="Observability requires an Enterprise Intelligence Platform license key"
-  body="Sign up for a free account to get a publicLicenseKey and start monitoring your agents."
+  body="Sign up for a free account to get a public license key and start monitoring your agents."
   ctaLabel="Create a free account"
   surface="docs_premium_observability"
 />