feat: sync Herdr plugin during wt update

[leesangb]

Summary

• sync an installed wt.herdr plugin to the wt release selected by wt update
• support both standalone and Homebrew-managed installations
• preserve locally linked plugins and plugins installed from other repositories
• make plugin sync best-effort so a completed wt update is not rolled back
• reinstall an already matching plugin when --force is used

Validation

• bun test
• bun run typecheck
• bun build src/index.ts --compile --outfile /tmp/wt-sync-plugin-build
• verified the installed Herdr plugin list JSON schema

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