pty/run answers with the command's output; pin cloud9 with 9ns concurrency and E.BUSY

The answer is now the header line (exit N, or exit N cut M when only the
last 64 KiB were kept, or exit N cut when its start scrolled out of the
history), then what the command printed: the screen text between its C and
D marks, which the marks handler pins so scrolling keeps them.

Also from review: a plain /log open honours its offset until follow is
written, so tail -n and less work; whitespace-only run lines are refused.

cloud9 is pinned at 82d8152c: 9ns keeps up to 32 requests in flight per
mount so a waiting read no longer freezes the rest of it, and E.BUSY
replaces the errno pardes spelled locally.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 8 insertions, 1 deletions

diff --git a/docs/fs.md b/docs/fs.md
index d46e3d21..90849a58 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -198,7 +198,14 @@ Closing the open is the only way back, as with rio's `consctl`.
 
 `pty/run` runs one line at a terminal's prompt and answers how it ended, on
 the same open (factotum's `rpc` shape): write the line, then read `exit N`
-once the command has ended and the shell is back at a prompt. It reads
+once the command has ended and the shell is back at a prompt, followed by
+what it printed. The output is what the screen showed between the command's
+start and end marks: stderr interleaved, `\r` progress collapsed to its last
+state, no colour; a full-screen program (vim, less) leaves none, and rows a
+program redrew above its start are missed. Only its last 64 KiB are kept,
+from a line start, and the header then reads `exit N cut M` (M bytes left
+out), or `exit N cut` when its start scrolled out of the history. The header
+is always the whole first line. It reads
 `busy` at once when a command is running, text is typed at the prompt or the
 shell has not drawn its first prompt; `error not run` when the shell refused
 the line without running it (a fish syntax error; the line is taken back off