test(e2e): phase 4 - compaction continuity tests

[sytone]

Summary

Phase 4 E2E tests covering compaction quality and continuity, grounded in real production failure modes observed in session history and informed by the Hermes agent framework (https://github.com/NousResearch/hermes-agent).

Closes / validates fix for #655.

What's tested

CompactionContinuityTests.cs — 7 tests:

Test	What it validates
SlashCompact_NotificationBubble_HasSystemMessageStyling	Compaction bubble uses chat-system-message testid — NOT styled as assistant message
SlashCompact_NextTurn_AgentRespondsNormally	Agent continues responding after /compact (regression: #602 fix #3)
PostCompaction_NewMessage_RespondsToNewTopic_NotSummaryTask	Latest message wins over ## Active Task in summary (Hermes SUMMARY_PREFIX guardrail)
PostCompaction_StopSignal_EndsInFlightWork	Stop signal after compaction does not resume in-flight work from summary
DoubleCompact_AgentContinuesAfterBothCycles	Two compaction cycles both produce notifications; agent still works after second
SlashCompact_NoCronConversationCreated	No rogue cron: conversation in sidebar (regression: #602 fix #1)
SlashCompact_NotificationAppearsBeforeNextResponse	Compaction boundary is ordered before the next assistant response, not after

New mock scripts

• COMPACTION_TASK_SEED — seeds a "monitoring" task context for guardrail tests
• COMPACTION_STOP_SIGNAL — returns a stop-acknowledged response

Background

See issue #655 for the full analysis of Hermes's approach vs BotNexus's current compaction gaps. These tests will fail on the current codebase in places where the gaps exist — that is intentional. They document the expected behaviour and will go green as #655's fixes land.

[sytone]

@copilot resolve the merge conflicts in this pull request