fix: @-reference button no-ops — read conversation via ctx.get, not the ctx property

Cordis only grants property reads for services declared in the plugin's
inject list; every other read throws "cannot get property X without
inject" (the app's own plugins read 'conversation' through ctx.get for
this reason). referenceInChat now resolves the conversation service lazily
via ctx.get('conversation') and no-ops when absent. Verified end-to-end in
headless Chrome: clicking the @file button on root/dir/file rows now
appends the at-relative-path mention to the composer draft with no
console errors.

src/context-types.ts

@@ -9,7 +9,9 @@
  * - httpServer: @deepseek-ai/dsh-host-webserver
  * - sessions: host side @deepseek-ai/dsh-session (SessionStore), client
  *   side the runtime ISessions list feed
- * - conversation: client side ui-conversation's IConversation (composer draft)
+ * - conversation: client side ui-conversation's IConversation (composer
+ *   draft), read lazily through `ctx.get` — cross-plugin service reads need
+ *   an inject declaration, so the direct property is never typed here
  * - loader: @cordisjs/plugin-loader (entry options)
  * - slots: the client runtime SlotsService
  * - effect: the DSH-vendored cordis lifecycle helper
@@ -116,7 +118,7 @@ export interface SidebarSessionInput {
   setDraft(text: string): void
 }
 
-/** The conversation service face (mirror of ui-conversation's IConversation). */
+/** The composer draft face the sidebar reaches through `ctx.get('conversation')`. */
 export interface SidebarConversation {
   input: {
     for(actx: Context): SidebarSessionInput
@@ -127,7 +129,6 @@ declare module 'cordis' {
   interface Context {
     httpServer: SidebarHttpServer
     sessions: SidebarSessionStore & SidebarSessionsService
-    conversation: SidebarConversation
     loader: SidebarLoader
     slots: SidebarSlotsService
     /**