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>

.changeset/multiline-input-paste.md

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+---
+'aicodeman': patch
+---
+
+A multi-line prompt sent through the mux (`POST /api/sessions/:id/input` with `useMux`, respawn prompts) now keeps its line breaks. `sendInput` used to strip every `\n` before the text reached tmux, so `"line one\nline two\r"` arrived as `line oneline two`. The lines now go in as one bracketed paste, the same paste a single-line prompt already uses, and the Enter after it submits them together; multi-line text without an Enter is pasted too instead of typed. A shell runs each line. The submit check that re-presses a stranded Enter does not yet recognise a multi-line paste in the composer, so `codeman agent send` and the TUI keep refusing or joining multi-line input.