[Feature]: Remove taskstoissues from the core command set

### Problem Statement

After the bundled GitHub Issues extension is available and the core `/speckit.taskstoissues` command has completed a deprecation period, retaining both implementations leaves GitHub-specific project-management behavior in the core SDD command set and creates duplicate maintenance surfaces.

The final migration stage should make the extension the sole owner of GitHub issue creation.

### Proposed Solution

After #4421 and #4422 have shipped in earlier minor releases, remove `taskstoissues` from the core command set.

The removal should cover all core-owned surfaces, including:

- The core command template.
- Core command inventories, ordering, and descriptions.
- Integration scaffolding and generated core command expectations.
- Agent-specific argument descriptions and tool declarations.
- Tests that assume `taskstoissues` is always installed as a core command.
- README, installation, integration, and upgrade documentation.

The bundled `github-issues` extension should become the sole implementation and continue to provide `speckit.github-issues.taskstoissues`.

Once the core namespace is free, add `speckit.taskstoissues` as a deprecated compatibility alias in the extension if the extension command and alias validation rules support it safely. The namespaced command remains canonical. Projects without the extension should no longer receive any tasks-to-issues command.

Document the breaking change prominently and provide an agent-consumable migration recipe:

```bash
specify extension add github-issues
specify integration upgrade
```

The exact refresh command should be verified against the supported upgrade flow before publication.

### Alternatives Considered

- **Retain the deprecated core command indefinitely:** Leaves provider-specific functionality and duplicate maintenance in core.
- **Auto-install the extension for everyone:** Preserves behavior but defeats the goal of making issue tracking opt-in.
- **Remove the core command without a compatibility alias:** Simpler, but unnecessarily disrupts users who have installed the replacement extension and still invoke the legacy name.

### Component

Specify CLI (initialization, commands)

### AI Agent (if applicable)

All agents

### Use Cases

1. New projects receive only the core SDD workflow unless they explicitly opt into GitHub issue tracking.
2. GitHub issue functionality can evolve and release independently as an extension.
3. Other issue-tracker providers can offer parallel extensions without replacing a core command.

### Acceptance Criteria

- [ ] This issue is implemented only after #4421 and #4422 have shipped in earlier minor releases.
- [ ] `taskstoissues` is removed from the core command templates and inventories.
- [ ] Fresh projects do not receive a tasks-to-issues command unless `github-issues` is installed.
- [ ] The extension remains installable and provides the canonical `speckit.github-issues.taskstoissues` command.
- [ ] A deprecated `speckit.taskstoissues` extension alias is provided if it can be registered safely after the core command is removed.
- [ ] Extension hooks using `before_taskstoissues` and `after_taskstoissues` continue to function when the extension command runs.
- [ ] Integration, preset, and extension tests are updated for the new opt-in behavior.
- [ ] Upgrade documentation clearly identifies the removal as breaking and provides migration commands.
- [ ] Related work, including #4370 and #2223, references the extension rather than the removed core command.
- [ ] The removal is included in a later minor release.

### Additional Context

This is stage 3 of the migration:

1. #4421 adds the bundled GitHub Issues extension.
2. #4422 deprecates the core command after the replacement ships.
3. This issue removes the command from core after the deprecation window.

Spec Kit's current release guidance treats the version as a release identifier rather than a promise that breaking changes require a new major version. The removal may therefore ship in a minor release, provided it is clearly identified and documented as breaking.

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

Start with the core command templates, command inventories, integration scaffolding, and generated command expectations mentioned in the issue. Then inspect agent-specific declarations plus integration, preset, and extension tests for assumptions that taskstoissues is always installed. Done means the core command is absent from fresh projects, the github-issues extension remains canonical, compatibility and hooks are handled safely, and migration documentation is updated.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.