refactor(cli): drop --format=json end-to-end

Companion to the compact preflight in #31 — the JSON format was
the structured alternative to the verbose text summary. With the
new compact text already on screen, no consumer was using the
JSON shape, and the abstract `BottlePlan.to_dict` was the
biggest piece of API surface no one is implementing against.

Removed:
  - `--format` CLI flag from `start` and `resume`.
  - `output_format` kwarg from `_launch_bottle`.
  - `BottlePlan.to_dict` abstract method.
  - `DockerBottlePlan.to_dict` (60-line dict builder).
  - The `_PlanView` dataclass — `print` was the only remaining
    caller, so the env-name computation is inlined.
  - `tests/integration/test_dry_run_plan.py` (JSON-shape
    integration test).
  - `tests/unit/test_cli_start_format.py` (flag-conflict unit).

Plan-introspection is still possible by reading the
`DockerBottlePlan` dataclass directly — fields like `image`,
`container_name`, `stage_dir`, `use_runsc` are all there. Tooling
that needs a stable wire shape can JSON-serialize the dataclass
themselves.

411 unit + integration tests pass.

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

claude_bottle/backend/__init__.py

@@ -73,13 +73,6 @@ 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):