feat: save IDE in config when selected during quickstart

[bespoyasov]

• Move config module to packages/core/ so the quickstart wizard can read/write config values (previously only available in packages/cli/). The CLI re-exports from core for backwards compatibility.
• Auto-select IDE when saved in config — if a previous wizard run saved the user's IDE choice, the install-plugins screen restores it automatically instead of showing the picker again. A new restoring phase and telemetry event (restored:<ide>) track this path.
• Save selected IDE to config on every manual selection so future runs can restore it.
• Add test infrastructure for config — createConfigDir() scaffold in packages/testing/ for isolated config dirs with Symbol.dispose cleanup. E2E session factory gains a config option to pre-seed config values.
• E2E coverage for the saved-IDE flow, verifying the picker is skipped and the wizard advances automatically.

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This will be handy in the granular CLI commands for MCP installation and agents setup