fix(extensions): ignore VS Code user extension folders

[SpencerJung]

Closes #50

Summary

• Stop runtime extension discovery from scanning VS Code's user extension directory (~/.vscode/extensions).
• Also stop scanning Cursor's user extension directory (~/.cursor/extensions) for the same reason.
• Keep SideX's own extension directory, bundled extension directory, dist/extensions, and repository extension fallbacks.

Issue Evidence

• Issue [BUG] #50 shows SideX touching a Java runtime under C:\Users\xxoo\.vscode\extensions\redhat.java-....
• Root cause: both Rust-side extension discovery and the Node extension host fallback included ~/.vscode/extensions in runtime search paths.
• After this change, SideX no longer scans VS Code/Cursor user-installed extensions by default.

Verification

• node --check src-tauri/extension-host/server.cjs — passed.
• git diff --check — passed.
• rust-analyzer LSP — blocked: rust-analyzer is not installed in this workflow shell.
• Rust compile/fmt checks — blocked per repository profile because cargo and rustc are not installed in this workflow shell.

Community Rules Review

• Focused PR for a single issue/concern.
• Kept runtime extension-host search roots consistent between Rust and Node fallback paths.
• No unauthorized version bump.
• No workflow/tracker/profile files included in the target repo diff.