feat: enable Chrome extension Browser Use on Linux

[cau1k]

Closes #33.

What

• ships a static Linux x86-64 native-messaging host with the Darwin protocol-v2 app-server proxy and legacy Browser Use relay
• enables Linux Chrome-extension discovery, installs both supported native-host manifests, and preserves the trusted bundled browser client
• mirrors desktop registry-v2 selection, runtime refresh, extension-origin checks, and persisted agent-mode defaults
• packages complete Rust dependency notices and documents Browser Use vs web mode vs unsupported Computer Use

The adjacent installer config remains only as the same standalone-install fallback used by the Darwin host. Desktop-managed installs use registry-v2.

Validation

• npm test — 126 Node tests + 57 Rust tests
• cargo clippy --locked --manifest-path native/chrome-extension-host/Cargo.toml --all-targets -- -D warnings
• npm run release:prod — built 26.707.31123-launcher.35
• packaged artifact smoke — static x86-64 ELF, protocol-v2 hello/ensure, registry-v2 app-server launch, desktop boot, all artifact contracts
• fresh desktop install — created both registry copies and the Google Chrome native-messaging manifest; cached host hash matched the packaged ELF
• real unpacked ChatGPT Chrome extension 1.2.27203.26575 — Chrome spawned the host, opened the side panel, launched app-server, and completed ping, getInfo, tab create/list/attach, CDP navigation, and runtime evaluation through the actual extension bridge

Current delivery target: Linux x86-64 with Google Chrome/compatible Chromium native messaging. Computer Use remains out of scope.