fix(tui,tool-tasks): finish removing dispose-ness from delivery

The TUI's referenced-prompt snapshot now rides the prompt's own
admission transaction instead of a pre-admission inject: while idle, a
one-shot prepended agent/prompt-submit wrapper appends the snapshot to
the allow decision's additionalContexts, so a blocking hook discards
the prompt and its attached context together instead of stranding the
snapshot in history for the next unrelated prompt. A prompt discarded
before admission releases the wrapper; steering keeps the inject path
since it bypasses admission and drains at the same boundary. The
session-reference snapshot adapter pinned the old context-before-prompt
order; the branch-wide order (prompt first, its contexts after) is now
asserted and the fixture re-recorded.

tool-tasks drops the last consumer of the removed thrown-disposed
contract: completion notices now inject unconditionally, which is
well-defined during owner teardown — the loop treats disposal like any
cancel, so the notice appends as durable idle context (persisted for
resume while the session is attached, dropped with the detached log
after). README pair and the owner-disposal tests state the new
delivery contract.

packages/ui/tui/tests/session-reference.snapshot.ts

@@ -24,10 +24,13 @@ class SnapshotAdapter extends LlmAdapter {
 
   async * stream(options: GenerateOptions): AsyncIterable<StreamChunk> {
     this.requests.push(options)
-    const [context, prompt] = options.messages.slice(-2)
+    // The snapshot rides the prompt's admission: the loop appends the
+    // prompt first, then its additional contexts (the branch-wide ordering
+    // for plugin-sourced context).
+    const [prompt, context] = options.messages.slice(-2)
     if (context?.role !== 'user' || prompt?.role !== 'user'
       || prompt.content[0]?.type !== 'text' || prompt.content[0].text !== 'Use @Source session') {
-      throw new Error('session reference context did not precede the direct user message')
+      throw new Error('session reference context did not follow the direct user message')
     }
     yield { type: 'block-start', index: 0, blockType: 'text' }
     yield { type: 'text-delta', index: 0, text: 'Combined reference request accepted.' }