docs: reword core-boundary note to keep the block/core grep gate clean

The README/AGENTS prose mentioned the literal `block/core` token when stating
that plugins must not import it. Reword to "the first-party core module" so
`grep -rn block/core` over the repo returns zero hits — P2 can use that grep as
a hard verification that no code (or doc) reintroduces a core dependency.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

AGENTS.md

@@ -4,8 +4,8 @@ Go module `git.dev.alexdunmow.com/block/pluginsdk`.
 
 **Purpose: this is the ONE plugin-facing artifact.** BlockNinja CMS plugins
 import this module and nothing else from the internal Go tree. Plugins must
-NOT import `block/core` — core exists only for code shared between first-party
-entities (cms, orchestrator, ninja CLI).
+NOT import the first-party `core` module — core exists only for code shared
+between first-party entities (cms, orchestrator, ninja CLI).
 
 ## The contract is proto, not Go
 

README.md

@@ -1,7 +1,7 @@
 # block/pluginsdk
 
 The **plugin-facing SDK** for BlockNinja CMS plugins. Plugins import THIS module,
-never `block/core`.
+never the first-party `core` module.
 
 ## What lives here
 
@@ -38,7 +38,7 @@ the SDK once host-side head/body injection lands — Phase 3.)
 - **NEVER use `replace` directives** in `go.mod`. Module resolution goes through the
   Gitea module proxy — to test local changes, tag and push a version.
 - All consumers are in-house — **no backwards-compatibility shims**.
-- Plugins import `block/pluginsdk/...`, never `block/core/...`.
+- Plugins import `block/pluginsdk/...`, never the first-party `core` module.
 
 ## Design