Add /selftest.extension core extension to test other extensions (#1758)

* test(commands): create extension-commands LLM playground sandbox

* update(tests): format LLM evaluation as an automated test runner

* test(commands): map extension-commands python script with timestamps

* test(commands): map extension-commands python script with timestamps

* test(commands): update TESTING.md to evaluate discovery, lint, and deploy explicitly

* test(commands): simplify execution expectations and add timestamp calculation

* fix(tests): address copilot review comments on prompt formatting and relative paths

* fix(tests): resolve copilot PR feedback regarding extension schema structure and argparse mutually exclusive groups

* feat(extensions): add core selftest utility and migrate away from manual tests sandbox

* fix(selftest): update command name array to match spec-kit validation schema

* fix(selftest): wrap arguments in quotes to support multi-word extension names

* update the command to be more meaningful

* fix: if the extension is discovery only, it should not be installable

* Address review comments for selftest documentation

* address review comments

* address review comments

* Update extensions/selftest/commands/selftest.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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Co-authored-by: Manfred Riem <15701806+mnriem@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>