fix(sync): keep installed skills when a registry cannot be read

[Fluzko]

What does this PR do?

If a configured registry cannot be read, cargo agents sync treats it as empty and silently deletes every skill it installed. A [[registry]] path on an unmounted drive or pointing at one package, or a deleted git copy, is enough; the same sync runs inside agent sessions.

Now such a registry is reported by name and sync removes nothing until it can be read again, while still installing the rest. A git registry never fetched points to cargo agents plugin sync <name>. A missing ~/.symposium/plugins/ is left out (nobody configured it). plugin list and plugin show report it too.

Verified by reproducing it before and after (moved or chmod 000 path registry, deleted git copy, session-start sync) and by tests, including a package path.

Take into account:

• While a registry stays unreadable (say, a recommendations registry never fetched), nothing is cleaned up and every sync warns.
• Only the registry directory is checked: an unreadable subdirectory still reads as empty.
• Whichever of this PR and feat(agent-plugins): compile global plugins and install for Claude Code #332 merges second puts compiled-plugin reaping behind the same check.

Disclosure questions

AI disclosure.

• The AI tool authored large parts of the code