feat(chrome): add Helium browser auto-detection

[zasdaym]

Summary

Add Helium as a supported Chromium-family browser anywhere agent-browser currently auto-detects Chrome and Brave for local launch, --auto-connect, profile reuse, and install-time browser detection.

Changes

• add Helium executable detection to the native CDP path on macOS, Linux, and Windows
• add Helium user-data directory discovery so --auto-connect, profiles, and --profile <name> can resolve Helium installs that use Chromium-style Local State
• extend scripts/postinstall.js so an installed Helium browser suppresses the local-browser warning
• update README, CLI help, docs site pages, and core skill references to describe --auto-connect and profile reuse in Chrome-family terms and explicitly include Helium
• add focused unit tests covering Helium user-data directory enumeration and find_chrome_user_data_dir() resolution

Test plan

• cargo test chrome:: --manifest-path cli/Cargo.toml

Notes

• This keeps Helium on the existing chrome engine path rather than introducing a new engine type.
• Windows support follows Helium's %LOCALAPPDATA%\\imput\\Helium layout, and Linux support includes the helium command plus Helium config directories.

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