feat(updater): fall back to OS system proxy when app proxy is disabled

[su-fen]

Summary

• The GitHub update path (release feed probe + tauri-plugin-updater download) used to force a direct connection via no_proxy() whenever the in-app proxy was disabled, ignoring OS proxy settings entirely — updates failed in environments where GitHub is unreachable directly.
• Update egress now resolves in order: app proxy when enabled (invalid config still fails fast) → OS system proxy (proxy env vars + macOS/Windows system proxy settings via reqwest's default system-proxy detection) → direct connection.
• Added client_builder_with_os_proxy_fallback() in services/system_proxy.rs as the dedicated entry point for the update path; build_updater() no longer calls .no_proxy() so the plugin's internal client picks up the same fallback (verified against plugin source; single shared reqwest 0.13.4 build in the lockfile keeps the system-proxy feature enabled for it).
• All other egress points (skills, ClawHub, MCP, local reverse proxy, …) keep the existing explicit no_proxy semantics; the now-unused async client_builder() is removed.

Testing

• cargo check --lib clean (no warnings).
• cargo test --lib system_proxy 8/8 and update::tests 5/5 pass.
• New test coverage: fallback builder accepts Disabled/Enabled modes and still fails fast on Invalid config (no silent downgrade to direct/system proxy).