review followup: empty-input guards, ~/ symmetry, CMD: filter

Addresses three concerns + one nit from the Phase 0 review pass.

executor.lua:
  - M.exec guards empty / whitespace-only cmd up front, returns
    "(empty command)" / -1 instead of running the wrapper on nothing.
  - On sentinel-parse failure with empty output (typical of shell
    parse errors — the syntax error itself escapes to the popen
    parent's stderr because 2>&1 is inside the unparsable subshell),
    surface "(no output — possible shell parse error)" rather than
    a silent empty frame.
  - extract_cmd_lines now skips whitespace-only / empty bodies; a
    bare `CMD: ` line in assistant output no longer turns into an
    "execute ''? [y/N]" prompt.
  - "what" comments cleaned in maybe_chdir.

router.lua:
  - path_like now matches `~` and `~/foo` so `~/scripts/build.sh`
    classifies as shell (was: ai). Restores symmetry with executor's
    maybe_chdir, which already expands `~` on `cd`.

repl.lua:
  - :exec and :ask trim args and renderer.status a usage line on
    empty rather than running an empty cmd / sending an empty turn
    to broker.

Regression: full prior smoke suite still passes — known_commands
shell paths, all maybe_chdir branches, CMD: extraction with non-empty
bodies, exec exit-code recovery, all router branches.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

executor.lua

@@ -16,9 +16,12 @@ local EXIT_SENTINEL = "__AISH_EXIT_4F8E91__"
 
 -- Execute a shell command.
 -- Returns: (output_string, exit_code).
--- exit_code == 0 on success; non-zero on failure; -1 if the sentinel could
--- not be parsed (e.g. popen itself failed, or output collided with the tag).
+-- exit_code == 0 on success; non-zero on failure; -1 on no-output / sentinel-
+-- parse failure (popen failed, empty cmd, shell parse error, sentinel collision).
 function M.exec(cmd)
+    if not cmd or cmd:match("^%s*$") then
+        return "(empty command)", -1
+    end
     local wrapped = string.format("(%s) 2>&1; echo %s$?", cmd, EXIT_SENTINEL)
     local handle, err = io.popen(wrapped, "r")
     if not handle then return ("popen failed: " .. tostring(err)), -1 end
@@ -27,6 +30,9 @@ function M.exec(cmd)
 
     local body, code = output:match("^(.-)" .. EXIT_SENTINEL .. "(%-?%d+)%s*$")
     if code then return body, tonumber(code) end
+    if output == "" then
+        return "(no output — possible shell parse error)", -1
+    end
     return output, -1
 end
 
@@ -40,10 +46,9 @@ function M.maybe_chdir(cmd)
               or cmd:match("^%s*cd%s+(.+)$")
     if not rest then return nil end
 
-    -- trim
     local target = rest:match("^%s*(.-)%s*$") or ""
 
-    -- defaults + ~ expansion (Phase 0: no $OLDPWD / `cd -`)
+    -- Phase 0: no $OLDPWD support, so `cd -` is not handled.
     if target == ""  then target = os.getenv("HOME") or "/" end
     if target == "~" then target = os.getenv("HOME") or "/" end
     if target:sub(1, 2) == "~/" then
@@ -60,7 +65,8 @@ function M.extract_cmd_lines(text)
     local cmds = {}
     for line in (text or ""):gmatch("[^\n]+") do
         local cmd = line:match("^CMD: (.*)$")
-        if cmd then cmds[#cmds + 1] = cmd end
+        -- Skip whitespace-only / empty bodies; "CMD: " alone is degenerate.
+        if cmd and cmd:match("%S") then cmds[#cmds + 1] = cmd end
     end
     return cmds
 end