guest: register marketplace for built-in plugins

[DarkaMaul]

Summary

• register the official Claude plugin marketplace for the built-in c profile
• register the same marketplace for the built-in rust profile
• allow the existing clangd-lsp and rust-analyzer-lsp plugin installs to resolve their marketplace

[hbrodin]

Thanks, Alexis, for fixing this! The rest looks good to merge once the requested regression coverage and platform validation pass. Both additions use the exact same official marketplace URL, which looks good.

One coverage request is inline. Review covered correctness, design, conventions, security, CLI usage, tests, docs, and comments; no other diff-introduced issues or diff noise were found. Neither Lima nor Firecracker VM integration was run during this review. Please record results for both platforms, keeping Firecracker's pre-existing omission of profile plugin installation as a separate follow-up. The CI failure tracked in #476 is excluded from this review.

[hbrodin]

Current CI failure is unrelated and can be disregarded for now.