feat(cli): add --format=json to start --dry-run for machine-readable plan

BottlePlan gains a to_dict method (abstract on the base, implemented
on DockerBottlePlan) returning a JSON-serializable view of the resolved
plan. `cli.py start --dry-run --format=json` prints it to stdout and
exits zero. --format=json without --dry-run is rejected — emitting JSON
during a real launch would race the y/N prompt.

The dry-run integration test now parses the JSON and asserts on
structured fields (agent, bottle, runtime, hosts sorted+deduped, etc.)
instead of regex-matching the human-readable preflight stdout. That
kills the magic-"8 hosts allowed" coupling — adding a new baked
default doesn't break the test.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/__init__.py

@@ -66,6 +66,13 @@ class BottlePlan(ABC):
     def print(self, *, remote_control: bool) -> None:
         """Render the y/N preflight summary to stderr."""
 
+    @abstractmethod
+    def to_dict(self, *, remote_control: bool) -> dict[str, object]:
+        """Return the plan as a JSON-serializable dict for machine
+        consumption (used by `start --dry-run --format=json`). The key
+        set is part of the CLI's user-facing contract — adding fields
+        is fine, renaming or removing is a breaking change."""
+
 
 @dataclass(frozen=True)
 class BottleCleanupPlan(ABC):