fix: make ripgrep resolution resilient in plugin cache

[lightsifter]

Summary

• Fix OMP/plugin-cache searches when the platform optional ripgrep package cannot be resolved.
• Keep bundled ripgrep preferred and use an executable rg on PATH as a bounded compatibility fallback.
• Preserve strict absolute executable validation for SIFT_LIGHT_RG_PATH.
• Regenerate Pi, OMP, MCP and Codex plugin artifacts for 1.0.5-1.

Verification

• bun run check:local: 560 pass, 3 skip, 0 fail.
• bun run pack:check passed for sift-light@1.0.5-1.
• Isolated generated OMP runtime search passed without borrowing repository node_modules.