Support concurrent Tailscale shares

[icodesign]

Summary

• return every ready provider install URL so the user can choose the preferred sharing route
• coordinate concurrent Tailscale Serve/Funnel startup across processes and auto-allocate non-conflicting HTTPS ports
• keep explicit port selection strict and improve invalid Tailscale JSON diagnostics

Validation

• cargo test (162 tests passed)
• cargo fmt -- --check
• git diff --check
• cargo clippy --all-targets -- -D warnings -A clippy::large-enum-variant
• npx --yes skills add . --list