fix: improve companion sync failure diagnostics

[shiny-code-bot]

Summary

• capture NSError domain/code for companion sync store failures
• log failed companion sync store role plus redacted error details
• extend companion sync partial-failure coverage so future iCloud/local split failures are easier to diagnose

Context

The iPad widget stale report on this host was caused by a stale/wrong installed macOS runtime. After reinstalling from a clean current worktree through the runtime gate, /Applications/Context Panel.app passed baseline and the local app-group companion document matched the iCloud companion document byte-for-byte after an installed refresh-agent run.

This patch intentionally does not change the companion sync storage path or publish behavior. It only makes any future partial publish failure explain which store failed and why.

Validation

• swift test --filter 'ContextPanelCoreTests.companionSyncStoreSetReportsPartialAndCompleteSaveFailures'
• scripts/context-panel-runtime-baseline.sh install --launch
• installed refresh agent --refresh-once
• verified local app-group and iCloud companion sync documents had matching timestamp, size, and SHA-256
• scripts/commit-gate.sh
• local review agents approved the final diff across GPT, Claude, and Gemini/Antigravity perspectives