docs(mcp): mark media tools MCP-only in index.ts (#294, media family)

The media tools — insert_image, replace_image, insert_footnote — are MCP-only
by design: the in-app AI-chat agent exposes no image or footnote tools, so there
is no second layer to unify into SHARED_TOOL_SPECS. A registry spec's
tier/catalogLine are in-app metadata and the catalog-partition test forbids a
spec without a live in-app tool, so forcing them into the registry would break
the invariant. They stay per-transport (inline in index.ts).

No behavior change — documentation only (adds the rationale above each tool so a
future migrator does not re-investigate why these are not shared).

Gate: mcp tsc 0 (comment-only change).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/mcp/src/index.ts

@@ -381,6 +381,10 @@ registerShared(SHARED_TOOL_SPECS.deleteNode, async ({ pageId, nodeId }) => {
 });
 
 // Tool: insert_image
+// MCP-only by design (NOT in the shared registry): the in-app AI-chat agent
+// exposes no image tools (insert/replace), so there is no second layer to unify
+// — a SHARED_TOOL_SPECS entry's tier/catalogLine are in-app metadata and the
+// catalog-partition test forbids a spec without a live in-app tool (#294).
 server.registerTool(
   "insert_image",
   {
@@ -426,6 +430,7 @@ server.registerTool(
 );
 
 // Tool: replace_image
+// MCP-only by design (see insert_image): no in-app equivalent, stays inline.
 server.registerTool(
   "replace_image",
   {
@@ -790,6 +795,8 @@ server.registerTool(
 );
 
 // Tool: insert_footnote
+// MCP-only by design (see insert_image): the in-app AI-chat agent exposes no
+// footnote tool, so there is no second layer to unify — stays inline (#294).
 server.registerTool(
   "insert_footnote",
   {