Remove retired ChannelRuntime direct callback endpoint

[eanzhao]

Summary

• Remove the retired direct platform callback route from ChannelCallbackEndpoints.
• Drop the direct_callback_retired diagnostic path that only existed for that route.
• Add route coverage proving ChannelRuntime no longer registers callback-style /api/channels/.../callback/... routes.
• Update runtime guidance and cutover docs to point only at NyxID relay ingress.

Fixes #671.

Validation

• dotnet test test/Aevatar.GAgents.ChannelRuntime.Tests/Aevatar.GAgents.ChannelRuntime.Tests.csproj --nologo
• dotnet test test/Aevatar.AI.Tests/Aevatar.AI.Tests.csproj --nologo --filter "FullyQualifiedName~NyxIdChatGAgentTests|FullyQualifiedName~NyxIdChatEndpointsCoverageTests"
• bash tools/ci/test_stability_guards.sh
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.31%. Comparing base (74d5afd) to head (ccb5fd2).
⚠️ Report is 3 commits behind head on dev.

@@            Coverage Diff             @@
##              dev     #673      +/-   ##
==========================================
- Coverage   82.31%   82.31%   -0.01%     
==========================================
  Files         932      932              
  Lines       59485    59485              
  Branches     7805     7805              
==========================================
- Hits        48965    48964       -1     
+ Misses       7160     7159       -1     
- Partials     3360     3362       +2     

Flag	Coverage Δ
ci	82.31% <ø> (-0.01%)	⬇️

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see 3 files with indirect coverage changes

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[eanzhao]

[codex] severity=major, category=arch

This mutates an existing ADR even though CLAUDE.md says ADRs are immutable and may only be superseded (docs/adr/ governance). Note ADR-0011 is already status: superseded (superseded by ADR-0013), so editing its "required order" step is both a governance violation and unnecessary — the authoritative contract now lives in ADR-0013. Recommend reverting this ADR-0011 edit entirely; if a current-runtime clarification is needed, put it in the runbook/canon docs instead.

[eanzhao]

[v4-pro] severity=minor, category=test

WebApplication created by builder.Build() implements IAsyncDisposable but is never disposed, leaking the host and its DI container. Fix: await using var app = builder.Build(); and make the test async. Note: the sibling test MapChannelCallbackEndpoints_ShouldRequireAuthorization_ForDiagnosticErrors has the identical undisposed pattern — if you fix this, fix both for consistency.