fix(extensions): normalize literal command calls in generated skills

[MahinurRahman]

Summary

Fixes #3451.

Extension commands can contain literal cross-command calls such as /speckit.memory-md.prepare-context. When those commands are installed as skills, the generated SKILL.md previously retained the dotted slash form even for integrations that require hyphenated skill names or a different invocation prefix.

This change normalizes literal skill invocations in the shared skill renderer and extension-manager skill registration path. It supports Codex ($speckit-*), Claude/Copilot (/speckit-*), and Kimi (/skill:speckit-*). Canonical command IDs, file paths, URLs, and ordinary non-skill command layouts are preserved.

New regression coverage exercises four integrations through both skill registration paths, verifies idempotence and path/URL preservation, and confirms ordinary command output remains dotted.

Testing

• Focused regression: 9 passed.
• Broader extension/preset/placeholder run: 1,201 passed, 3 skipped, 23 deselected. Symlink-named tests were excluded because this Windows account cannot create symbolic links (WinError 1314); the failures were test setup failures, not assertion failures.
• CLI smoke test: initialized a non-interactive Codex skills project with the PowerShell environment and installed a local two-command extension. The generated skill invoked $speckit-invocation-smoke-finish and the referenced skill existed.
• git diff --check passed.

AI disclosure: Codex authored the implementation, tests, documentation, and validation on behalf of MahinurRahman. The work was performed autonomously; no human line-by-line review or external-agent execution is claimed.