⚙️ [Maintenance]: Remove old spec-kit prompts, scripts, and templates (#281)

The repository still contained files from the old spec-kit workflow that
is no longer in use. The instructions system under
`.github/instructions/` now handles Copilot guidance. Removing these
legacy files reduces clutter and prevents confusion about which files
are active.

- Fixes #280

## Removed files

### Copilot instructions

The old `.github/copilot-instructions.md` file from spec-kit has been
removed. Copilot instructions are now managed through files in
`.github/instructions/`.

### Prompt files

All 8 spec-kit prompt files under `.github/prompts/PSModule.*.prompt.md`
have been removed:
- `PSModule.analyze.prompt.md`
- `PSModule.clarify.prompt.md`
- `PSModule.constitution.prompt.md`
- `PSModule.implement.prompt.md`
- `PSModule.plan.prompt.md`
- `PSModule.pr.prompt.md` — this prompt has been converted to a PSModule
organization custom agent
- `PSModule.specify.prompt.md`
- `PSModule.tasks.prompt.md`

### Specify scripts and templates

The entire `.specify/` directory has been removed, including:
- 5 PowerShell scripts (`check-prerequisites.ps1`, `common.ps1`,
`create-new-feature.ps1`, `setup-plan.ps1`, `update-agent-context.ps1`)
- 5 Markdown templates (`agent-file-template.md`,
`constitution-template.md`, `plan-template.md`, `spec-template.md`,
`tasks-template.md`)

No action is required from users — these files were internal tooling
only.

Author: MariusStorhaug