docs: the README and skill answer what dogfooding asked

A first-time user through a mount could not find how to get a terminal,
read path++text as a typo, did not know > on data replaces only the
addressed range, which regexps addr takes, that lock fails fast, what
errors is, where an unknown exec word goes (typed into a terminal, not an
error), that new panes are +New and a column's last pane leaves one, or
which 9ns mount other processes can see. The README (still 45 lines),
docs/fs.md and the skill now say.

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

1 files changed, 15 insertions, 1 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 6eb56537..c54cfc4d 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -59,6 +59,14 @@ private namespace:
 9ns --unix "$PARDES_9P" -- sh -c 'cat "$NINE_MOUNT/index"'
 ```
 
+A `9ns --unix` mount lives in the private namespace of the command it runs,
+and nothing outside that command sees it. The mount everyone on the machine
+shares is the registry one, `9ns --mntgen` (default `/mnt/9p`): every running
+editor posts itself there, so `/mnt/9p/pardes/<pid>/` is that editor's tree
+for any process. A new pane made through `pane/new` is a scratch named
+`<dir>/+New` until it is given a name, and closing a column's last pane
+leaves such a `+New` in its place (`Delcol` closes the column).
+
 For [Linux v9fs](https://www.kernel.org/doc/html/latest/filesystems/9p.html),
 use `version=9p2000,cache=none,access=any` and `trans=unix`, or `trans=tcp`
 with `port=5640`. Set `uname`, `dfltuid`, and `dfltgid` for the local user.
@@ -194,7 +202,13 @@ same tree without leaving the process.
 - a line written to `exec` is a middle click: a command word from
   `src/builtins.zig` (`Save`, `Del`, `New`, `Newcol`, `Mount NAME DIAL`,
   `Unmount NAME`, `Dump`, `Restore`, `Msg TEXT`, `Find`, `Grep`, `Tty`, ...),
-  or anything else, which runs in the pane's terminal.
+  or anything else, which is typed into a terminal: the pane itself when it
+  is a terminal at its prompt, otherwise a terminal in the pane's directory
+  that is at its prompt, and failing both a new one made below the last
+  column. That is not an error, whatever the shell makes of the line, and a
+  misspelled builtin word ends up there too (whether it should is an open
+  question, docs/open-questions.md). `echo Tty > pane/<n>/ctl` makes a
+  terminal in that pane's directory outright.
 
 The root's pair clicks at the active pane and `/pane/<n>/look` and
 `/pane/<n>/exec` at that pane. Blank lines are skipped, and every other line