fix: isolate external path searches for #106

[lightsifter]

Summary

• Keep searches rooted at an explicitly selected external path by default.
• Preserve explicit scope expansion and existing in-repository behavior.
• Anchor ripgrep globs and excludes to the selected external root.
• Update release metadata and generated plugin artifacts for 1.0.3-3.

Verification

• bun run check
• bun run check:local (529 passed, 3 skipped)
• bun run pack:check
• Focused external-path regression checks (63 passed)