fix(desktop): migrate legacy updates to dual channels

[pallyoung]

What changed

• split Desktop updates into a legacy Runtime channel and a modern Shell channel
• keep Shell 0.1.1 and its updater metadata immutable while delivering Runtime 0.5.9
• publish Shell 0.1.2 through desktop-channel-modern.json / modern.yml
• disable differential and web-installer downloads in Shell 0.1.2+
• show Shell 0.1.1 users a one-time manual 0.1.2 installer notice
• validate migration and future Runtime-only releases across both channels
• extend Desktop/CLI acceptance and promotion gates for migration reports

Root cause

Shell 0.1.1's electron-updater download path stalls even for tiny full-file probes, so blockmap and bootstrap fallbacks cannot reliably migrate that installed Shell. The legacy channel must avoid requesting a Shell download at all.

Impact

Existing Shell 0.1.1 installations receive Runtime 0.5.9 automatically, then install Shell 0.1.2 once manually. Fresh 0.1.2 installations use the modern channel and full installer downloads. Windows builds remain unsigned when release input windows_signing=false, so Windows will show an Unknown publisher warning.

Validation

• pnpm changeset:validate
• pnpm ci:typecheck
• targeted Desktop, Web, channel, artifact, and workflow tests
• targeted Biome check
• pnpm ci:build

Full local lint/test are affected by the Windows checkout's CRLF conversion and existing platform-specific tests; PR CI is the authoritative Linux gate.