Standardize timestamps to UTC across ChatMessage and dispatch paths

[PureWeen]

Context

PR #375 revealed a DateTime.UtcNow vs DateTime.Now mismatch in the orchestration dispatch path (N5 finding). The fix was to use DateTime.Now to match ChatMessage.Timestamp convention, but this is a band-aid — the codebase uses mixed conventions.

Current state

• ChatMessage.Timestamp uses DateTime.Now (local time)
• PendingOrchestration.StartedAt uses DateTime.UtcNow
• Resume path has manual ToLocalTime() conversion
• Dispatch path now uses DateTime.Now after the N5 fix

Proposed fix

1. Standardize all persisted/logical timestamps to DateTimeOffset.UtcNow
2. Convert to local time only in UI display layers
3. Update ChatMessage.Timestamp, PendingOrchestration.StartedAt, and all comparison sites
4. Ensure events.jsonl timestamps are parsed consistently

Risk

This is a data migration — existing persisted sessions have local timestamps in events.jsonl. Need a migration path or dual-parsing.

Priority

Low — current code works correctly after the N5 fix, but the mixed convention is a recurring source of bugs.

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Pull request created: #751

Generated by Agent Fix

[github-actions]

Fix Implemented — PR #387

What Changed

Standardized all timestamp fields from DateTime to DateTimeOffset with DateTimeOffset.UtcNow, eliminating the mixed DateTime.Now/DateTime.UtcNow convention that caused cross-timezone comparison bugs.

Key changes across 29 files (16 production, 13 test):

• ChatMessage.Timestamp, ToolActivity, PendingOrchestration.StartedAt, ReflectionCycle timestamps, AgentSessionInfo.CreatedAt → DateTimeOffset
• All 10 ChatMessage factory methods → DateTimeOffset.UtcNow
• Removed ToLocalTime() workarounds in dispatch/resume paths
• UI display uses .LocalDateTime.ToShortTimeString()
• ChatDatabase boundary conversion with backward compat for old local-time SQLite rows
• 6 new behavioral tests for UTC standardization

Test Results

✅ 3543 passed, 0 failed, 0 skipped

Self-Review (3-model adversarial)

Reviewers: Claude Opus 4.6, Claude Sonnet 4.6, GPT-5.3 Codex

3 findings fixed (by consensus):

1. ChatDatabase backward compat (3/3): Old SQLite rows stored local time — now detects DateTime.Kind instead of assuming UTC
2. TryParse fallback (2/3): DateTimeOffset.TryParse clobbered fallback with default on parse failure — now uses temp variable
3. DateTime.UtcNow consistency (3/3): Two sites used DateTime.UtcNow - pending.StartedAt instead of DateTimeOffset.UtcNow

CI Dispatched

• ✅ verify-build dispatched
• ✅ polypilot-integration (smoke) dispatched

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Generated by Agent Fix for issue #386 · ● 74.6M · ◷