docs: record the open question of where an unknown command word runs

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

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+# Open questions
+
+Decisions deliberately left open, with what is known so far, so they can be
+picked up without redoing the research.
+
+## Where does an unknown command word run?
+
+Raised 2026-09-27 in the review of pardes against acme (Plan 9 source at
+`~/05-genizah/principia-softwarica`). Undecided.
+
+**Today.** A middle-click, an `exec` write or a tag word that is not a builtin
+is typed into a terminal pane: the pane itself when it takes a command line,
+else a shell for the pane's directory (`Pardes.execute`, `ttyForDir` in
+`src/pardes.zig`). The command shares that shell's cwd, environment, history
+and aliases, and its output lands in the terminal.
+
+**acme.** An external command runs as its own process: stdin from
+`/dev/null`, stdout and stderr to the directory's `+Errors` window, `$winid`
+and `%` set for it (`editors/acme/exec.c`, `run()` and its callers around
+:1180-1300 and :1425). No shell state is shared between commands.
+
+**What each costs.**
+
+* Typing into a shell: interactive programs work, and so do aliases and
+  functions. But every command depends on that shell's state (is it busy,
+  which directory it is in, what was typed at its prompt), and a misspelled
+  builtin silently becomes a shell command. The 9P `exec` file cannot report
+  that the command failed.
+* A process per command: the misspelling hazard goes away at the root, the
+  editor knows each command's exit status and output, and `exec` could answer
+  the way `pty/run` does. But aliases, shell functions and interactive
+  programs need a terminal of their own, and output goes to a `+Errors`-style
+  buffer rather than a live terminal.
+
+**Related.** `pty/run` (see `docs/fs.md`) already gives an agent the second
+behaviour inside a chosen terminal: a line in, `exit N` and its output out.
+The planned root `ctl` (session builtins) and pane `ctl` (pane builtins)
+refuse unknown words, whatever is decided here; the question is only what
+`exec` and the middle-click do with them.