refactor(bottle): extract claude_docker_argv from exec_claude

PRD 0021 chunk 1. The tmux split-pane helpers (chunk 2+) need
the same docker-exec argv that `exec_claude` builds — including
the `--append-system-prompt-file <path>` flag the bottle's
provisioner copies into place. Extract the argv construction
into a pure `claude_docker_argv(argv, *, tty)` method so both
foreground (`subprocess.run`) and tmux paths
(`tmux respawn-pane …`) build from the same source.

`exec_claude` becomes a one-liner that runs subprocess.run on
the argv. No behavior change; 472 unit tests pass (7 new for
the pure builder).

claude_bottle/backend/docker/bottle.py

@@ -28,7 +28,15 @@ class DockerBottle(Bottle):
         self._prompt_path = prompt_path_in_container
         self._closed = False
 
-    def exec_claude(self, argv: list[str], *, tty: bool = True) -> int:
+    def claude_docker_argv(
+        self, argv: list[str], *, tty: bool = True,
+    ) -> list[str]:
+        """Return the full `docker exec` argv for running claude in
+        this bottle. Public so callers that want to spawn claude
+        somewhere other than the dashboard's foreground (e.g.,
+        `tmux split-window` / `tmux respawn-pane` from the dashboard
+        when `$TMUX` is set) can build on the same command without
+        duplicating the `--append-system-prompt-file` plumbing."""
         full_argv = list(argv)
         if self._prompt_path:
             full_argv.extend(["--append-system-prompt-file", self._prompt_path])
@@ -36,7 +44,12 @@ class DockerBottle(Bottle):
         if tty:
             cmd.append("-it")
         cmd.extend([self.name, "claude", *full_argv])
-        return subprocess.run(cmd, check=False).returncode
+        return cmd
+
+    def exec_claude(self, argv: list[str], *, tty: bool = True) -> int:
+        return subprocess.run(
+            self.claude_docker_argv(argv, tty=tty), check=False,
+        ).returncode
 
     def exec(self, script: str) -> ExecResult:
         # Pipe via stdin to `sh -s` so the caller never has to worry

tests/unit/test_docker_bottle.py

@@ -0,0 +1,84 @@
+"""Unit: DockerBottle's argv builder (PRD 0021 chunk 1).
+
+`claude_docker_argv` is the pure helper that `exec_claude` and the
+PRD-0021 tmux helpers both build on. It encodes two non-trivial
+rules — the optional `--append-system-prompt-file` flag and the
+optional `-it` for TTY mode — that we lock down here so the tmux
+path can rely on identical behavior.
+"""
+
+from __future__ import annotations
+
+import unittest
+
+from claude_bottle.backend.docker.bottle import DockerBottle
+
+
+def _bottle(prompt_path: str | None = None) -> DockerBottle:
+    return DockerBottle(
+        container="claude-bottle-dev-abc",
+        teardown=lambda: None,
+        prompt_path_in_container=prompt_path,
+    )
+
+
+class TestClaudeDockerArgv(unittest.TestCase):
+    def test_minimal_argv_no_prompt(self):
+        argv = _bottle().claude_docker_argv([])
+        self.assertEqual(
+            ["docker", "exec", "-it", "claude-bottle-dev-abc", "claude"],
+            argv,
+        )
+
+    def test_appends_passed_args_after_claude(self):
+        argv = _bottle().claude_docker_argv(
+            ["--dangerously-skip-permissions", "--continue"],
+        )
+        self.assertEqual(
+            ["docker", "exec", "-it", "claude-bottle-dev-abc", "claude",
+             "--dangerously-skip-permissions", "--continue"],
+            argv,
+        )
+
+    def test_appends_prompt_file_flag_when_set(self):
+        argv = _bottle("/home/node/.claude-bottle-prompt.txt").claude_docker_argv(
+            ["--dangerously-skip-permissions"],
+        )
+        self.assertEqual(
+            ["docker", "exec", "-it", "claude-bottle-dev-abc", "claude",
+             "--dangerously-skip-permissions",
+             "--append-system-prompt-file",
+             "/home/node/.claude-bottle-prompt.txt"],
+            argv,
+        )
+
+    def test_no_prompt_flag_when_none(self):
+        argv = _bottle(None).claude_docker_argv(["--continue"])
+        self.assertNotIn("--append-system-prompt-file", argv)
+
+    def test_empty_prompt_string_is_treated_as_no_prompt(self):
+        # Matches the existing exec_claude behavior: falsy
+        # prompt_path means "skip the flag." The synth path in
+        # dashboard.py relies on this when metadata is missing.
+        argv = _bottle("").claude_docker_argv(["--continue"])
+        self.assertNotIn("--append-system-prompt-file", argv)
+
+    def test_tty_false_drops_it_flag(self):
+        argv = _bottle().claude_docker_argv([], tty=False)
+        self.assertEqual(
+            ["docker", "exec", "claude-bottle-dev-abc", "claude"],
+            argv,
+        )
+
+    def test_caller_argv_not_mutated(self):
+        # `claude_docker_argv` builds `full_argv` from a copy, so a
+        # caller passing a long-lived list (e.g., the dashboard's
+        # _claude_args fixture) doesn't get extra flags appended to
+        # it on subsequent calls.
+        original = ["--continue"]
+        _bottle("/x").claude_docker_argv(original)
+        self.assertEqual(["--continue"], original)
+
+
+if __name__ == "__main__":
+    unittest.main()