Add logical new conversation command

[louis4li]

Issue

Closes #3611

Scope

• Remove /reset from retained-history clear command handling.
• Keep /clear scoped to clearing the current conversation retained transcript window.
• Add /new as a private-chat command that rotates the physical channel chat to a fresh logical conversation actor.
• Add actor-owned ChannelConversationThreadGAgent state/command/event contract for durable active logical conversation routing.

Verification

• dotnet test test/Aevatar.GAgents.ChannelRuntime.Tests/Aevatar.GAgents.ChannelRuntime.Tests.csproj --nologo --filter "ConversationDispatchMiddlewareTests|ConversationGAgentRetainedHistoryClearTests|ChannelConversationTurnRunnerTests" - 170 passed
• bash tools/ci/test_stability_guards.sh - passed
• bash tools/ci/architecture_guards.sh - failed on existing NyxID conformance declaration: src/Aevatar.Mainnet.Host.Api/Hosting/MainnetHostBuilderExtensions.cs differs from declared revision; this PR does not modify that file

Notes

• No product endpoint check was run because this change is channel runtime command routing/actor behavior, covered by focused runtime tests.

Generated with Claude Code.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-09T13:04:51.559952Z	dd93ad5	PR opened

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💡 Codex Review

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Reviewed commit: dd93ad5093

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.47%. Comparing base (358bc70) to head (6cca55a).
⚠️ Report is 19 commits behind head on feature/integrate.

@@                  Coverage Diff                  @@
##           feature/integrate    #3612      +/-   ##
=====================================================
+ Coverage              83.45%   83.47%   +0.01%     
=====================================================
  Files                   1709     1709              
  Lines                 197728   197842     +114     
  Branches               23735    23740       +5     
=====================================================
+ Hits                  165016   165145     +129     
+ Misses                 21670    21661       -9     
+ Partials               11042    11036       -6     

Flag	Coverage Δ
ci	83.47% <ø> (+0.01%)	⬆️

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