Workflow Templates: add three curated built-ins

[AbigailDeng]

Summary

• Publish exactly three curated built-in workflow templates with stable IDs, revisions, and product ordering.
• Add localized typed metadata, expected input/output, tags, and truthful LLM/human-interaction requirements.
• Validate embedded YAML through the canonical runtime parser and Studio authoring, schema, and graph contracts, with focused draft-run coverage.
• Depends on Add read-only workflow template catalog API #2973.

Verification

• env DOTNET_EnableWriteXorExecute=0 dotnet test test/Aevatar.Studio.Tests/Aevatar.Studio.Tests.csproj --nologo --no-restore --disable-build-servers -p:IsTestProject=true --verbosity minimal --filter FullyQualifiedName~BuiltInWorkflowTemplateCatalogTests|FullyQualifiedName~EmbeddedWorkflowTemplateCatalogQueryPortTests|FullyQualifiedName~WorkflowTemplateEndpointsTests|FullyQualifiedName~StudioInfrastructureServiceCollectionExtensionsTests: passed (22 tests)
• env DOTNET_EnableWriteXorExecute=0 dotnet build src/Aevatar.Studio.Hosting/Aevatar.Studio.Hosting.csproj --nologo --no-restore --disable-build-servers: passed
• bash tools/ci/test_stability_guards.sh: passed
• bash tools/docs/lint.sh: passed (77 files)
• bash tools/ci/query_projection_priming_guard.sh: passed
• bash tools/ci/architecture_guards.sh: blocked at protobuf lint because buf is unavailable locally; every preceding architecture check passed

Risks

• Local architecture verification lacks only buf lint; CI supplies the repository-pinned buf version.
• Dependency PR Add read-only workflow template catalog API #2973 must land first.

Closes #2968

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.45%. Comparing base (5426eff) to head (6545a89).

@@           Coverage Diff           @@
##              dev    #2976   +/-   ##
=======================================
  Coverage   83.45%   83.45%           
=======================================
  Files        1376     1376           
  Lines      112732   112732           
  Branches    14366    14366           
=======================================
  Hits        94084    94084           
  Misses      12096    12096           
  Partials     6552     6552           

Flag	Coverage Δ
ci	83.45% <ø> (ø)

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