[Feature]: Deprecate the core taskstoissues command

### Problem Statement

`/speckit.taskstoissues` is currently presented as a core Spec Kit command even though it performs GitHub-specific issue-tracker integration. Once the bundled GitHub Issues extension from #4421 is available, users should be guided toward the extension before the core command is removed.

Removing the command without a deprecation period would provide no transition window for existing projects, automation, presets, or dependent extensions.

### Proposed Solution

After #4421 has shipped, mark the core `/speckit.taskstoissues` command as deprecated while preserving its behavior for this stage.

When invoked, the command should display a concise warning that:

- The core command is deprecated and will be removed in a future minor release.
- Its replacement is the bundled `github-issues` extension.
- Users can migrate with:

```bash
specify extension add github-issues
/speckit.github-issues.taskstoissues
```

The deprecation must not automatically install or enable the extension. Existing command behavior, safety checks, deduplication, and lifecycle hooks should remain unchanged until the removal stage.

Update user-facing command inventories and documentation to distinguish the deprecated core command from the recommended extension command.

### Alternatives Considered

- **Deprecate before adding the extension:** Rejected because the warning would point users toward a replacement that is not yet available.
- **Silently document the migration without an invocation warning:** Easy to miss for users who do not regularly read release notes.
- **Remove the command in the same release:** Provides no practical migration window for existing users or dependent extensions.

### Component

Specify CLI (initialization, commands)

### AI Agent (if applicable)

All agents

### Use Cases

1. Existing users learn how to migrate while their current command still works.
2. Preset and extension authors have time to update dependencies and hook references.
3. Teams can schedule the extension installation before a later upgrade removes the core command.

### Acceptance Criteria

- [ ] This issue is implemented only after #4421 has shipped.
- [ ] Invoking the core `speckit.taskstoissues` command displays an actionable deprecation warning.
- [ ] The warning identifies `speckit.github-issues.taskstoissues` as the replacement and includes the installation command.
- [ ] The deprecated core command retains its existing behavior.
- [ ] `before_taskstoissues` and `after_taskstoissues` hooks continue to work.
- [ ] Core command documentation and inventories clearly mark the command as deprecated.
- [ ] Migration documentation covers existing projects, integrations, presets, and extensions.
- [ ] #4370 and other dependent work can update their dependency target to the extension command.
- [ ] The change is included in a minor release after the extension release.

### Additional Context

This is stage 2 of the three-stage migration:

1. #4421 adds the bundled extension while retaining core behavior.
2. This issue deprecates the core command.
3. A follow-up issue will remove it from core after the deprecation window.

Related discussion: #4370, especially https://github.com/github/spec-kit/issues/4370#issuecomment-5525946904.

After #4421 ships, start at the core `/speckit.taskstoissues` CLI entry point and inspect the command inventories and user-facing documentation. Verify the existing behavior, safety checks, deduplication, and `before_taskstoissues`/`after_taskstoissues` hooks before adding the warning. Done means the command still works, the migration commands and deprecation status are documented, and dependent work can target the extension.

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