fix: rewrite extension skill command references

[NgoQuocViet2001]

Description

• rewrite standalone portable slash-dot references in extension skill bodies using the active agent's native invocation syntax
• share the invocation renderer with hook execution so Codex, Kimi, Cline, and slash-based integrations stay consistent
• preserve URL, filesystem-path, Markdown-link, HTML-attribute, and query-path references
• document the portable extension command-reference behavior

Closes #3451

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

Additional validation:

• uv run pytest tests/test_extension_skills.py tests/test_extensions.py tests/integrations/test_base.py tests/integrations/test_integration_kimi.py tests/integrations/test_integration_zcode.py -q (527 passed, 14 skipped)
• uvx ruff check src/specify_cli/extensions/__init__.py tests/test_extension_skills.py
• git diff --check

The sample-project coverage installs an extension into temporary Codex, Kimi, and Claude projects and verifies both rewritten invocations and preserved path/link references.

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (describe below)

Codex (GPT-5, autonomous) analyzed the issue, implemented and iteratively tested the change on behalf of @NgoQuocViet2001. The operator requested this contribution but did not manually author or line-by-line review the patch.