Fix --dev extension agent symlinks

[NgoQuocViet2001]

Description

Fixes #2451.

specify extension add --dev now asks extension registration to link generated agent command files when possible. The registrar writes rendered output into an extension-local .specify-dev cache and points the agent-facing command file at it, while falling back to a normal copy when symlink creation is unavailable. The same option is threaded through extension skill registration, and unregister now removes symlinked command entries too.

Testing

• Tested locally with PYTHONIOENCODING=utf-8 PYTHONUTF8=1 uv run specify --help
  • Note: plain uv run specify --help on this Windows cp1252 console hit an existing banner UnicodeEncodeError; the UTF-8 run passed.
• Ran existing tests with uv sync --extra test && uv run pytest tests/test_extensions.py
• Tested with a sample project (if applicable): covered by tests/test_extensions.py::TestExtensionAddCLI::test_add_dev_links_copilot_agent_when_supported
• uv run pytest tests/integrations/test_integration_subcommand.py -k "extension_add or switch_migrates_extension_commands or switch_migrates_copilot_skills_extension_commands"
• uvx ruff check src/specify_cli/agents.py src/specify_cli/extensions.py src/specify_cli/__init__.py
• git diff --check

AI Disclosure

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

This PR was implemented with OpenAI Codex assistance. Codex helped identify the regression path, make the code changes, add tests, run validation, and draft this PR; I reviewed the resulting diff and test output before submission.