fix(plugin): inline mcpServers in plugin.json so the MCP server actually loads

/reload-plugins reported '0 plugin MCP servers'. Root cause: plugin.json had
"mcpServers": "./.mcp.json" — a string path, which is neither a valid inline
object nor a recognized reference (per docs, plugin MCP servers are a root
.mcp.json OR an inline object in plugin.json), so it parsed to zero servers.

Inline the mcpServers object directly in plugin.json and remove the separate
.mcp.json. The user_config substitution syntax was already correct
(plugins-reference: values substitute as ${user_config.KEY} in MCP configs).

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

plugin/README.md

@@ -33,7 +33,7 @@ On install you'll be asked for:
 
 ## What gets wired
 
-- `.mcp.json` → the `scribe` MCP server at `<base URL>/mcp` (Bearer auth).
+- `plugin.json` `mcpServers` → the `scribe` MCP server at `<base URL>/mcp` (Bearer auth).
 - `hooks/hooks.json` → SessionStart hook (`hooks/scribe_session_context.sh`),
   **fail-open**: if Scribe is unreachable it injects nothing and never blocks
   the session.