* fix: prevent extension self-install from deleting source dir (#2990)
`specify extension add <path> --dev --force` permanently deleted the
extension directory without registering it when the source path resolved
to the extension's own install location (`.specify/extensions/<id>`).
With `--force`, `install_from_directory()` removed the existing
installation (the source) and then `shutil.copytree()` tried to copy from
the now-deleted directory, destroying it and crashing.
Add a guard that fails fast with a clear ValidationError when the resolved
source path equals the install destination, before any destructive
operation runs. Includes a regression test asserting the directory and its
contents survive.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
* fix: harden extension self-install guard
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Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
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