fix(install): never delete hooks settings.json still points at, nor a foreign manifest

Uninstall aborts instead of removing hook files whenever settings.json could
not be parsed or rewritten — leaving them while the entries remain is #471 on
every session start. The POSIX and PowerShell uninstallers exit 1 on a missing
node for the same reason.

hooks/package.json is the one HOOK_FILES entry that is not ours: install now
leaves another plugin's manifest alone and uninstall deletes only the exact
one we ship. Temp downloads move to a 0700 mkdtemp so a pre-planted symlink
cannot capture the init script we then execute.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SqmbqWDtNexzDfH5V3BMRz