One rule for failing builtins: the write fails, one err, no msg

A builtin that failed through a ctl failed the write and logged its err,
but through look, exec, tagexec or a column's exec it only said so on the
message row, logged as a msg, and the write succeeded. Now every click
write runs its builtin as a ctl line does (ctl.captured): a failure fails
the write, with its words (EINVAL for malformed input, else EIO or what
the words name), logs one err and no msg or announcement. get's Modified
and a look miss are shown but logged once, as their err. A control
character's refusal names its reason, a failed click reads back nothing,
and look never reads back a pane closed since. The special case for a
cut-short Edit through exec goes (the rule covers it); fs.md's table of
exceptions becomes the one rule, as does the skill.

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

1 files changed, 6 insertions, 3 deletions

diff --git a/.agents/skills/pardes-9p/SKILL.md b/.agents/skills/pardes-9p/SKILL.md
index 4f903d5d..9e540de8 100644
--- a/.agents/skills/pardes-9p/SKILL.md
+++ b/.agents/skills/pardes-9p/SKILL.md
@@ -154,9 +154,12 @@ an open that wrote, until its next write); open again, or seek to 0, to read
 it again. Each command line runs once whole, however a mount cuts a big write; a last
 line with no newline runs when the open closes, and an Edit block still open
 then fails there (an `err`: ``unmatched `{'``, or an a/c/i text with no `.`
-line), changing nothing. A command that fails is reported in
-the editor, not as a write error, so inspect the resulting pane, index, message
-or screen; only a malformed line fails the write itself. A word no builtin
+line), changing nothing. One rule: a builtin that fails, whether through a
+ctl, look, exec, tagexec or a column's exec, fails the write (EINVAL for a
+malformed line, else EIO or an errno that fits) and logs one `err` with the
+reason, no `msg`. A look that finds nothing is no failure: it answers
+nothing and logs one `err`. A command line run in a command pane is judged
+by its `exit` record. A word no builtin
 knows (a typo included) is a command line: written at a terminal at its
 prompt it is typed into that shell; from anywhere else it runs as a command
 pane, a terminal whose child is the root ctl's `Shell` ($SHELL, else /bin/sh, unless set)