main: non-interactive -p/--prompt one-shot mode (closes #4)

Adds `aish -p "<text>"` for Unix-pipeline composability:

  tail app.log | aish -p "any anomalies?"
  aish -p "summarize: $(curl -sS https://...)"

The flag bypasses repl.lua entirely. On invocation:

  1. Stdin: when not a TTY, read to EOF and prepend to the prompt as a
     fenced block. ffi.libc.isatty(0) gates the read so interactive
     `aish -p "..."` (no pipe) doesn't hang.
  2. Resolve config.models[config.default_model].
  3. Stream broker.chat_stream replies to stdout; finalize with newline.
  4. Exit 0 on success, 1 on broker error, 2 on arg / config error.

Behavior NOT in -p mode (kept simple per the issue's "no repl.lua
involvement"):
  - No MCP, no tool loop, no Norris, no routing, no memory injection.
  - "CMD:" lines in the reply are printed verbatim, NOT executed —
    callers can grep / pipe them as they wish.

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

ffi/libc.lua

@@ -38,6 +38,10 @@ int poll(struct pollfd *fds, unsigned long nfds, int timeout);
    enforcement via LOCK_EX | LOCK_NB — fail-fast if another aish
    process holds the lock. */
 int flock(int fd, int operation);
+
+/* TTY detection for non-interactive mode (`aish -p`). Returns 1 if the
+   fd refers to a terminal, 0 otherwise (sets errno on error). */
+int isatty(int fd);
 ]]
 
 local C = ffi.C
@@ -174,4 +178,9 @@ function M.flock(fd, op)
     return false, ffi.string(C.strerror(C.__errno_location()[0]))
 end
 
+-- ---------------------------------------------------------------- isatty
+function M.isatty(fd)
+    return C.isatty(fd) == 1
+end
+
 return M

main.lua

@@ -1,6 +1,6 @@
 -- main.lua — entry point
 -- Phase 0: arg parsing, config load, REPL start.
--- See docs/PHASE0.md §4, §10.
+-- See docs/PHASE0.md §4, §10. -p one-shot mode lands per issue #4.
 
 -- Make project modules and the vendored dkjson resolvable from the repo root.
 -- Run aish with the repo root as cwd; PTY-relative resolution lands later.
@@ -9,7 +9,13 @@ package.path = "./?.lua;./vendor/?.lua;" .. package.path
 local USAGE = [[
 aish — AI-augmented conversational shell.
 
-Usage: luajit main.lua [--config <path>] [--help]
+Usage:
+  luajit main.lua [--config <path>] [--help]            -- interactive REPL
+  luajit main.lua -p "<prompt>" [--config <path>]       -- one-shot, print + exit
+
+In -p mode, if stdin is not a TTY it's read as additional context and
+prepended to the prompt as a fenced block — composes with Unix pipes:
+  tail app.log | aish -p "any anomalies?"
 
 Config resolution order (PHASE0.md §10):
   1. --config <path>
@@ -29,6 +35,13 @@ local function parse_args(argv)
         elseif a == "--help" or a == "-h" then
             out.help = true
             i = i + 1
+        elseif a == "-p" or a == "--prompt" then
+            out.prompt = argv[i + 1]
+            if not out.prompt then
+                io.stderr:write("aish: -p requires a prompt argument\n")
+                os.exit(2)
+            end
+            i = i + 2
         else
             io.stderr:write("aish: unrecognized argument: " .. a .. "\n")
             os.exit(2)
@@ -63,6 +76,48 @@ local function load_config(opts)
           .. table.concat(candidates, ", ") .. ")")
 end
 
+-- One-shot mode: read non-TTY stdin (if any), compose prompt, stream
+-- broker reply to stdout, exit. Bypasses repl.lua entirely — no REPL,
+-- no MCP, no tool loop, no Norris. The model's reply is printed
+-- verbatim (including any "CMD:" lines, which are NOT executed in
+-- this mode by design — the user can pipe-grep them as they wish).
+local function run_one_shot(config, user_prompt)
+    local libc   = require("ffi.libc")
+    local broker = require("broker")
+
+    local composed = user_prompt
+    if not libc.isatty(0) then
+        local piped = io.read("*a") or ""
+        if piped ~= "" then
+            composed = "```\n" .. piped .. "\n```\n\n" .. user_prompt
+        end
+    end
+
+    local model_name = config.default_model
+    local model_cfg  = config.models and config.models[model_name]
+    if not model_cfg then
+        io.stderr:write(("aish: default_model '%s' not found in models{}\n")
+                        :format(tostring(model_name)))
+        os.exit(2)
+    end
+
+    local messages = { { role = "user", content = composed } }
+    local got_any = false
+    local ok, err = broker.chat_stream(model_cfg, messages,
+        function(kind, payload)
+            if kind == "text" and payload and payload ~= "" then
+                io.write(payload); io.flush()
+                got_any = true
+            end
+        end)
+    if not ok then
+        if got_any then io.write("\n") end
+        io.stderr:write("aish: broker error: " .. tostring(err) .. "\n")
+        os.exit(1)
+    end
+    if got_any then io.write("\n") end
+end
+
 local function main(argv)
     local opts = parse_args(argv or {})
     if opts.help then io.write(USAGE); return end
@@ -70,6 +125,11 @@ local function main(argv)
     local config, config_path = load_config(opts)
     io.stderr:write(("aish: loaded config from %s\n"):format(config_path))
 
+    if opts.prompt then
+        run_one_shot(config, opts.prompt)
+        return
+    end
+
     local repl = require("repl")
     repl.run(config)
 end