fix(native-chat): only seed single-line unsubmitted launch drafts

The unsubmitted-delivery branch seeded on every draft delivery, which also
caught the agent-session-fork path whose prompt is multi-line scraped context.
The chat send pre-clears the TUI with Ctrl+U (kill-to-start-of-LINE), so a
multi-line prefill cannot be fully cleared and its earlier lines would glue
onto the next message. The GitHub work-item draft this feature targets is a
bare issue URL, so narrowing costs it nothing.

Also assert the composer retires the seed after a send — deleting that call
previously failed no test.

src/renderer/src/components/native-chat/NativeChatComposer.test.tsx

@@ -32,7 +32,9 @@ const mocks = vi.hoisted(() => ({
   sendNativeChatMessageVerified: vi.fn(),
   trackPendingSend: vi.fn(),
   setDraft: vi.fn(),
-  draftScopeKeys: [] as string[]
+  draftScopeKeys: [] as string[],
+  clearNativeChatLaunchDraft: vi.fn(),
+  markNativeChatLaunchDraftAdopted: vi.fn()
 }))
 
 vi.mock('../../store', () => {
@@ -40,8 +42,8 @@ vi.mock('../../store', () => {
     dictationState: 'idle',
     settings: { voice: { enabled: false }, nativeChatSessionOptions: {} },
     updateSettings: vi.fn(),
-    clearNativeChatLaunchDraft: vi.fn(),
-    markNativeChatLaunchDraftAdopted: vi.fn()
+    clearNativeChatLaunchDraft: mocks.clearNativeChatLaunchDraft,
+    markNativeChatLaunchDraftAdopted: mocks.markNativeChatLaunchDraftAdopted
   }
   const useAppStore = (selector: (value: typeof state) => unknown) => selector(state)
   useAppStore.getState = () => state
@@ -203,6 +205,22 @@ describe('NativeChatComposer', () => {
     expect(mocks.trackPendingSend).toHaveBeenCalledWith(mocks.sendHandle, 'pending-1')
   })
 
+  it('retires the launch-draft seed once a send clears the TUI input line', () => {
+    render(
+      <NativeChatComposer
+        terminalTabId="tab-1"
+        paneKey="tab-1:leaf-1"
+        targetPtyId="pty-1"
+        agent="codex"
+      />
+    )
+    expect(mocks.clearNativeChatLaunchDraft).not.toHaveBeenCalled()
+
+    act(() => mocks.fieldProps?.onSend?.())
+
+    expect(mocks.clearNativeChatLaunchDraft).toHaveBeenCalledWith('tab-1')
+  })
+
   it('keeps the draft scope anchored to the pane while the PTY reconnects', () => {
     const view = render(
       <NativeChatComposer

src/renderer/src/lib/agent-launch-prompt-delivery.test.ts

@@ -101,6 +101,21 @@ describe('deliverLaunchPromptToAgentTab', () => {
     expect(mocks.seedNativeChatLaunchPrompt).not.toHaveBeenCalled()
   })
 
+  it('does not seed a launch draft for multi-line content', async () => {
+    // The chat send pre-clears the TUI with Ctrl+U (kill-to-start-of-LINE), so a
+    // multi-line prefill (e.g. scraped session-fork context) would leave earlier
+    // lines behind to glue onto the next message.
+    await deliverLaunchPromptToAgentTab({
+      tabId: 'fork-tab',
+      agent: 'codex',
+      content: 'Forked from session\n\nhttps://example.test/context',
+      submit: false,
+      forcePaste: false
+    })
+
+    expect(mocks.seedNativeChatLaunchDraft).not.toHaveBeenCalled()
+  })
+
   it('does not seed a launch draft for submitted, unsupported, or empty content', async () => {
     await deliverLaunchPromptToAgentTab({
       tabId: 'submit-tab',

src/renderer/src/lib/agent-launch-prompt-delivery.ts

@@ -43,9 +43,12 @@ export function deliverLaunchPromptToAgentTab(args: {
       text: content,
       createdAt: Date.now()
     })
-  } else if (submit !== true) {
+  } else if (submit !== true && !content.includes('\n')) {
     // Why: an unsubmitted draft lives only in the TUI input buffer; seed the
     // chat-composer copy so the context isn't invisible in the GUI view.
+    // Single-line only: the chat send pre-clears the TUI with Ctrl+U
+    // (kill-to-start-of-LINE), so a multi-line prefill (e.g. scraped fork
+    // context) would leave earlier lines to glue onto the next message.
     seedNativeChatLaunchDraftForAgentTab({ tabId, agent, text: content })
   }