fix: preserve extension authors in generated skills

[philo-x]

Description

An extension declaring author: acme-corp currently produces skills attributed to github-spec-kit. Pass the declared author through command registration and skill rendering, including aliases, dev-mode output, and restoration of extension skills after preset removal. Core generation and extensions with a missing or empty author retain the existing github-spec-kit default.

Closes #4458.

Testing

• Added 16 attribution cases covering both extension skill generators, normal/dev output, aliases, quoted/multiline authors, and missing/empty authors. Against unmodified main (4a7341a9), the 8 explicit-author cases fail; all pass with this change.
• Added an author assertion to the preset-removal extension restoration test.
• Final regression run: .venv/bin/python -m pytest tests/test_extensions.py tests/test_extension_skills.py tests/test_presets.py tests/integrations -q -k 'not test_ps_variant_prefixed_with_powershell_launcher' — 4202 passed, 5 skipped, 1 deselected.
• The excluded PowerShell launcher test fails because neither pwsh nor powershell is installed locally. Reproduced the same failure against unmodified main in an isolated checkout.
• uvx ruff@0.15.0 check src tests and git diff --check: passed.
• Verified .venv/bin/specify --help and a fresh CLI init followed by extension add --dev: the extension skill attributes acme-corp, while the core plan skill retains github-spec-kit.
• Independently checked single-agent registration and aliases for Claude and Codex.

AI Disclosure

• I did use AI assistance.

Implementation, regression tests, validation, commits, and this PR description were authored by OpenAI Codex (model: GPT-6, autonomous) on behalf of @philo-x. Each commit includes an Assisted-by: trailer.