[Feature]: Add `specify init --dry-run` preview

## Context

This issue supersedes [#1035](https://github.com/github/spec-kit/issues/1035), which requested `specify init . --dry-run` and was automatically closed for inactivity rather than resolved.

`specify init` can merge into an existing directory with `--force`, potentially changing generated project artifacts. Users need a reliable way to inspect the exact initialization plan before allowing writes.

## Proposal

Add:

```text
specify init --dry-run
```

The dry-run must use the same resolution and planning path as a real initialization, but perform no project filesystem mutations. It should preview the outcome for the requested directory and the selected integration, script type, preset, and extensions.

## Expected output

For every planned artifact, report:

- action: `create`, `overwrite`, `preserve`, `skip`, or `conflict`;
- project-relative path;
- provenance: core, integration, preset, workflow, or extension.

Provide a human-readable summary and `--json` output for CI/tooling.

## Acceptance criteria

- `specify init --dry-run` never creates, modifies, deletes, or registers project files.
- Its plan matches a subsequent real `specify init` invocation with the same options.
- Existing-directory conflicts are reported without needing `--force`.
- The preview accounts for core assets, integrations, presets, bundled/local extensions, and the bundled workflow.
- URL extensions are not installed during preview; any unresolved/download-required result is reported explicitly.
- Human-readable and JSON output have regression coverage, including a no-write assertion.

## Non-goals

- Running coding-agent commands or workflows.
- Replacing the existing `specify workflow run --dry-run` behavior.

Start at the existing `specify init` command and trace its resolution and planning path, since the issue names no files or tests. Compare dry-run planning with a real initialization, then add regression coverage for human-readable and JSON output, including a no-write assertion; done means all listed artifact sources and URL-extension outcomes are represented without filesystem mutations.

索引モデルが issue の本文から書いたものです。