[SFenton]

Summary

• version portable Copilot CLI user defaults with GPT-5.6 Sol as the interactive default
• merge only managed settings keys while preserving unrelated local configuration
• reject conflicting managed-key edits and retain exact rollback behavior
• document settings installation and restart behavior

Validation

• node --test tests/install.test.mjs (9 passed)
• npm test (233 passed, 13 skipped)
• real installer upgrade verified ~/.copilot/settings.json uses gpt-5.6-sol

[SFenton]

CI note: all shared repository suites passed on Ubuntu, macOS, and Windows. The projects integration job failed on external project-policy drift (prompt-start-enabled, relocated contract expectations, and stale inventory assumptions); the latest main workflow run, https://github.com/SFenton/copilot-config/actions/runs/35288178992, already fails the same projects job while all shared suites pass. This PR does not modify project manifests or those external repositories.