fix(input): keep the line breaks of a multi-line mux prompt, pasted as one block

TmuxManager.sendInput() removed every `\n` before the text reached tmux, so a
multi-line prompt sent with `useMux` (or a multi-line respawn prompt) arrived as
one line with its lines run together: `"echo A\necho B\r"` ran `echo Aecho B`.

The text half of a submit has gone in as one bracketed paste
(buildPasteTextCommands) since the Codex paste-burst fix, and inside the paste
markers Claude Code and Codex insert a pasted line break into the composer
instead of submitting. So the `\n` is now kept (`\r\n` loses only its `\r`) and
the existing paste carries the lines; the Enter stays a separate tmux call.
Multi-line text without an Enter is pasted too rather than typed with
`send-keys -l`, where every line break would reach the pane as a key.
Single-line input is unchanged.

The test-mode gate moves from the top of sendInput() into inputRunner(), as
readPaneRows() does, so a test subclass can record the commands sendInput()
would run instead of pinning the branch shapes on the source.

SubmitVerifier still compares the composer's last glyph line with the prompt
minus its newlines, so it does not reliably recognise a multi-line paste and
may not re-press an Enter that Claude Code ignored. `agent send` and the TUI
therefore keep refusing or joining multi-line input; the docs that described
the stripping now describe the paste and that limit.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/cli-agent.ts

@@ -129,16 +129,18 @@ export function sendPromptFromArgs(words: readonly string[]): { text: string } |
 }
 
 /**
- * Why `send` refuses this text, or `undefined` when it is printable. The composer
- * takes one line; the server strips `\r`/`\n` but everything else below 0x20 (and
- * DEL) reaches the pane as a keypress. None of that is a prompt.
+ * Why `send` refuses this text, or `undefined` when it is printable. A line break is
+ * refused for the reason below; everything else below 0x20 (and DEL) reaches the pane
+ * as a keypress. None of that is a prompt.
  */
 export function inputRefusal(text: string): string | undefined {
   if (text.length === 0) return 'refusing: empty input (use `interrupt` for ESC, `send <id> ""` is never a prompt)';
-  // The composer is one line: the server strips newlines, which silently joins the
-  // lines into one prompt, and a tab reaches the pane as a keypress (claude: mode toggle).
+  // The server pastes a multi-line prompt as one block, but the check that re-presses
+  // a stranded Enter cannot reliably recognise that block in the composer, so a wait
+  // could burn its timeout on a prompt that never went out. A tab reaches the pane as
+  // a keypress (claude: mode toggle).
   if (/[\n\r\t]/.test(text)) {
-    return 'refusing: input must be a single line (the composer strips newlines and would join your lines) — join them yourself, or write a file into the workspace and send its path';
+    return 'refusing: input must be a single line (a multi-line paste cannot be confirmed as submitted) — join them yourself, or write a file into the workspace and send its path';
   }
   // C0, DEL and C1 (U+0080–U+009F: an 8-bit CSI is still a CSI to a terminal).
   // eslint-disable-next-line no-control-regex