URI handler: always-both agent-host picker util (dark)

[amitjoshi438]

Summary

• add a telemetry-free agent-host QuickPick utility that preserves detector order and always keeps uninstalled hosts selectable
• localize GitHub Copilot CLI, Claude Code, and installed/not-installed descriptions
• add desktop integration coverage for both host states and cancellation

Validation

• npm run translations-export
• node node_modules/gulp/bin/gulp.js lint
• npm run compile-tests
• npm test (110 passing)
• npm run build (2 expected LiquidJS warnings)
• npm run test-desktop-int (926 passing)

Notes

• Dark development only; no handler wiring, telemetry, probing, ECS-gate, reload, or global-state changes. Keep this PR in draft pending OII sign-off.
• The shared not-installed description mirrors GI-A (URI handler: agentic-install telemetry events + constants + strings (dark) #1653) exactly at URI_HANDLER_STRINGS.DESCRIPTIONS.AGENT_HOST_NOT_INSTALLED, with a TODO to drop the duplicate on rebase after GI-A merges. The host display names and installed descriptions are GI-B-owned.