feat: add MCP client plugin (dsh-mcp-client)

Connects to an external MCP server and registers its tools on
ctx.tools. Supports stdio (child process) and Streamable HTTP
transports. Credential-shaped env vars are scrubbed before forwarding
to child processes.

- Plugin lifecycle: connect, sync tools, re-sync on ToolListChanged,
  dispose unregisters and closes
- Full JSDoc on all exports (@param/@returns on functions)
- 100% per-file coverage (apply lifecycle, args coercion, env scrubbing)
- Config catalog regenerated

scripts/gen-config-catalog.ts

@@ -537,6 +537,15 @@ function walkSchemaExpr(
       }
       return
     }
+    // A union of objects (discriminated union config): collect keys from all
+    // variants. Each variant is visited the same way as an intersect element.
+    if (method === 'union' && call.arguments[0] && ts.isArrayLiteralExpression(call.arguments[0])) {
+      for (const el of call.arguments[0].elements) {
+        const part = unwrapExpr(el)
+        if (ts.isCallExpression(part)) { visit(part); continue }
+      }
+      return
+    }
     // A chained refinement (`z.object({…}).default(…)` etc.): the keys live on
     // the call the chain hangs off — keep unwrapping toward it.
     const base = unwrapExpr(call.expression.expression)