runner: pipe prompts > 64KB via stdin to avoid OS argv limit

Cobb's seed-cleanup job hit OSError [Errno 7] Argument list too long with
a 577KB prompt. Linux ARG_MAX is typically 128KB-2MB depending on kernel +
env; passing the full prompt as 'claude -p <PROMPT>' fails for big jobs.

Fix: detect prompt size > 64KB threshold, omit the positional prompt
argument from the CLI invocation and pipe via subprocess.run(input=...)
instead. claude -p reads the prompt from stdin when no positional given.

System prompt + flags still pass as CLI args (those stay small).

clawdforge/runner.py

@@ -36,14 +36,18 @@ class Runner:
         files: list[str] | None = None,
         timeout_secs: int | None = None,
     ) -> RunResult:
-        cmd = [
-            self.claude_bin,
-            "-p",
-            prompt,
-            "--output-format",
-            "json",
-            "--model",
-            model or self.default_model,
+        # Linux ARG_MAX caps a CLI argument around 128KB-2MB depending on
+        # kernel + env. Long prompts (recipe corpora, big curate jobs) blow
+        # past it. For prompts > 64KB we pipe via stdin instead.
+        ARG_MAX_THRESHOLD = 64 * 1024  # bytes; leaves headroom for env+other args
+        use_stdin = len(prompt.encode("utf-8")) > ARG_MAX_THRESHOLD
+
+        cmd = [self.claude_bin, "-p"]
+        if not use_stdin:
+            cmd.append(prompt)
+        cmd += [
+            "--output-format", "json",
+            "--model", model or self.default_model,
         ]
         if system:
             cmd += ["--append-system-prompt", system]
@@ -66,6 +70,7 @@ class Runner:
                 text=True,
                 timeout=timeout,
                 cwd=str(cwd),
+                input=prompt if use_stdin else None,
             )
         except subprocess.TimeoutExpired as e:
             duration_ms = int((time.monotonic() - started) * 1000)