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>

repl.lua

@@ -126,9 +126,17 @@ function M.run(config)
                     i, t.role, t.content:gsub("\n", " ")))
             end
         end,
-        exec = function(args) run_shell(args) end,
-        ask  = function(args) ask_ai(args)    end,
-        help = function() io.write(HELP)      end,
+        exec = function(args)
+            args = (args or ""):match("^%s*(.-)%s*$")
+            if args == "" then renderer.status("usage: :exec <cmd>"); return end
+            run_shell(args)
+        end,
+        ask  = function(args)
+            args = (args or ""):match("^%s*(.-)%s*$")
+            if args == "" then renderer.status("usage: :ask <text>"); return end
+            ask_ai(args)
+        end,
+        help = function() io.write(HELP) end,
     }
 
     -- Main loop.