repl: background CMD&: with handle/poll (closes #8)

Builds, long-running network calls, and file watches no longer block
the turn. A new "CMD&: <cmd>" marker (analogue of CMD:) tells the REPL
to spawn the command in the background, return immediately, and poll
for completion between user inputs.

Process model: shell-wrapped to avoid needing fork()/execv() FFI.

  nohup sh -c '(<cmd>) > <log> 2>&1; echo $? > <status>' </dev/null
       >/dev/null 2>&1 & echo $!

The child is reparented to init; we hold only the PID and the path to
the .status sidecar. Completion is detected by the .status file
existing (the wrapper writes it as its last act). No waitpid needed —
the child isn't ours after the popen subshell exits.

Storage: <history.dir>/bg/<id>.log + <id>.status. The directory is
created lazily at startup (mkdir -p). Requires history.dir to be
configured; without it CMD&: emits an error status and the model
sees an "[bg failed to start]" exec-output note.

check_bg_done() runs at the top of each main-loop iteration alongside
check_every_due(). When a job is detected as exited, the REPL:
  - emits a status line "[bg:<id> exited <code>, <bytes>, <secs>s wall] <cmd>"
  - appends the same string to ctx as exec output, so the model sees
    the completion on its next turn (natural follow-up: "ok the build
    finished; let me check the log")

Meta surface:
  :bg-spawn <cmd>       start a bg job directly (no AI needed; also
                        useful for testing without depending on the
                        model emitting CMD&:)
  :bg-list              show running/done jobs (id, pid, state, runtime, cmd)
  :bg-output <id>       dump the log file to stdout
  :bg-kill <id>         SIGTERM (note: only delivers if the PID is
                        still the actual command — long-lived shells
                        may need pkill by name)

Scope (deliberately limited for v1):
  - No callback-mode readline: bg completion detection is pre-prompt,
    not mid-readline. If a build finishes while the user is typing,
    notification comes when they hit Enter.
  - Permission policy DSL (#9) does NOT apply to CMD&: — the
    asynchronous gating model wasn't designed for the y/N flow.
    Filed as follow-up if needed.
  - Norris not extended: helpers.exec_cmd is still synchronous; the
    planner doesn't dispatch bg jobs.
  - Plan mode interaction: CMD&: in plan mode emits "PLAN: & <cmd>"
    and a "[plan] would bg-run: <cmd>" exec-output note, no spawn.

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

executor.lua

@@ -127,11 +127,21 @@ end
 -- Extract `CMD: ` lines from an assistant response per the §6 broker contract.
 -- The "CMD: " prefix is a §3 substrate invariant: exact prefix, single space,
 -- start-of-line only. Leading whitespace before CMD: does NOT match.
+-- "CMD&: " lines are issue #8 background variants — extracted separately so
+-- repl.lua can route them to the bg spawner instead of the synchronous gate.
 function M.extract_cmd_lines(text)
     local cmds = {}
     for line in (text or ""):gmatch("[^\n]+") do
         local cmd = line:match("^CMD: (.*)$")
-        -- Skip whitespace-only / empty bodies; "CMD: " alone is degenerate.
+        if cmd and cmd:match("%S") then cmds[#cmds + 1] = cmd end
+    end
+    return cmds
+end
+
+function M.extract_cmd_bg_lines(text)
+    local cmds = {}
+    for line in (text or ""):gmatch("[^\n]+") do
+        local cmd = line:match("^CMD&: (.*)$")
         if cmd and cmd:match("%S") then cmds[#cmds + 1] = cmd end
     end
     return cmds