docs: finish dropping the stripped-newline rule from the input docs

The first commit left the old rule standing in places: the gotchas list in
extending-codeman.md still said newlines were stripped and that
"echo A\necho B\r" ran `echo Aecho B`, and so did both READMEs and the
skill's big-input section. Those now describe the paste.

Newlines are only kept on the mux path, so the docs that describe the paste
say "with useMux": without it a multi-line body is not a plain prompt and is
written straight into the pane.

Cron keeps rejecting multi-line prompts, but its docs and the code comments
in cron-service.ts, schemas.ts and respawn-controller.ts gave a reason this
change removes (writeViaMux joining lines, a newline submitting in Ink). The
reason that remains is the submit check, which cannot recognise a
multi-line paste in the composer, and for cron's paste mode the newlines
reaching the CLI as keys.

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

src/cron/cron-service.ts

@@ -97,7 +97,7 @@ const CRON_PSEUDO_FS_TREES: readonly string[] = ['/proc', '/sys', '/dev'];
 /** Sync blocklist for the create/update workingDir gate (no settings extras). */
 const CRON_WORKING_DIR_BLOCKED_TREES: readonly string[] = [...DEFAULT_BLOCKED_TREES, ...CRON_PSEUDO_FS_TREES];
 
-/** Prompt delivery is single-line only (writeViaMux/Ink constraint). */
+/** Prompt delivery is single-line only: the submit check cannot recognise a multi-line paste. */
 const HAS_NEWLINE = /[\r\n]/;
 
 /** The three session calls prompt delivery needs, so it can be tested without a PTY. */