feat: add --force flag and local-install make target

[gzb1128]

Summary

• Add --force / -f flag to plugin install and plugin update for overwriting existing symlinks
• Add make local-install and make uninstall-local targets (installs to ~/.local/bin, no sudo)
• Update README with local-install instructions and note that building from source avoids macOS Gatekeeper issues
• Add TestLinker_CreateSymlinks_ForceOverwrite test

Verification

• go test ./... — all pass
• go vet ./... — clean
• go build — clean