[codex] Speed up sidebar dashboard loading

[RandyZ]

Summary

• Warm dashboard data after DataManager.initialize() so sidebar activation can use an already-started load instead of waiting for the first view render.
• Coalesce passive dashboard reads while preserving write-triggered refresh semantics with a queued refresh path.
• Catch file watcher refresh failures and add regression coverage for concurrent reads plus mutation-during-refresh races.

Validation

• pnpm test test/extension/services/dataManager.test.ts
• pnpm test test/extension/providers/dashboardViewProvider.test.ts test/extension/providers/webviewMessageHandler.test.ts test/extension/services/dataManager.test.ts
• pnpm test (21 files, 151 tests)
• pnpm run build

Notes

Local git push was unavailable because HTTPS credentials were not configured in this environment, so the branch was published through the GitHub connector contents API. The local worktree still contains unrelated pre-existing unstaged changes that are not included in this PR.