bug(agy): --model ignored, EXTRA_ARGS appended after --print, and --add-dir workspace not passed

### Summary

In `src/specify_cli/integrations/agy/__init__.py`, `AgyIntegration.build_exec_args()` currently hardcodes ignoring the `model` parameter:

```python
def build_exec_args(
self,
prompt: str,
*,
model: str | None = None,
output_json: bool = True,
) -> list[str] | None:
# agy does not support --model or JSON output; both params are ignored
args = [self._resolve_executable(), "--print", prompt]
self._apply_extra_args_env_var(args)
return args
```

### Issues Observed

1. **`--model` is supported by `agy` CLI**:
Modern Antigravity CLI (`agy`) supports `--model ` (e.g. `agy --model claude-sonnet-4-6 -p "..."` or `agy --model gemini-3.1-pro-high -p "..."`). Because `model` is omitted from `build_exec_args`, stage-level model tiering defined in workflows (such as `.specify/workflows/...`) fails to route to the desired model, silently falling back to the user's default local model.

2. **Working directory / workspace flag**:
When `agy` is launched in headless `--print` mode, it does not automatically bind the current directory unless `--add-dir ` is passed, causing it to report:
```text
You do not currently have an active workspace open.
By default, scratch files and projects are located at: ~/.gemini/antigravity-cli/scratch
```

3. **Flag positioning before `--print`**:
Any extra args appended *after* `--print ` are treated by `agy`'s CLI parser as part of the positional prompt text rather than CLI flags. Flags (such as `--model`, `--add-dir`, `--dangerously-skip-permissions`, `--print-timeout`) should precede `--print`.

### Proposed Fix

In `AgyIntegration.build_exec_args()`:

```python
def build_exec_args(
self,
prompt: str,
*,
model: str | None = None,
output_json: bool = True,
) -> list[str] | None:
args = [self._resolve_executable()]
if model:
args.extend(["--model", model])
self._apply_extra_args_env_var(args)
args.extend(["--print", prompt])
return args
```

And in `AgyIntegration.dispatch_command()`, pass `--add-dir` with `project_root` when invoking the subprocess so `agy` recognizes the project repository workspace.

Happy to submit a PR with tests if this looks good to the maintainers!

Start in src/specify_cli/integrations/agy/__init__.py by reading AgyIntegration.build_exec_args() and dispatch_command(). Verify that model and extra arguments precede --print, and that project_root is passed through --add-dir; done means the generated command routes the requested model, preserves flags, and binds the repository workspace, with tests covering these cases.

索引モデルが issue の本文から書いたものです。