AIGTWY-4878: exercise managed schema-pointer CUJ3 for both agents

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Summary

• Exercise AIGTWY-4878 with the real published CUJ3 managed config: ug_e2e.models, ug_e2e.tools, and ug_e2e.skills schema pointers for both Claude and Codex.
• Require exact in-scope model inventories and accessible decoys in other schemas. Invoke the configured default and additional discovered compatible model for each agent, complete a file task, and record native model identity.
• Check both MCP services and both downloaded skills with native tool calls, tool results, and completed answers containing values withheld from prompts. Use separate run IDs for each skill. Inspect ug mcp list, ug status, and managed skill inventory.
• Preserve the earlier provider-override checks as managed_fixture cases; they are separate from CUJ3.

Verification

• 63 integration contract tests passed.
• CUJ3 marker collection selects exactly one Claude case and one Codex case.
• Ruff check and format passed for tests/; Python syntax compilation passed.
• Live CUJ3 execution is pending. The workspace still needs the App/OAuth M2M connection, UC fixtures, and the published config from AIGTWY-4877/4878: add reusable CUJ catalog fixtures #949. No valid local CUJ3 workspace profile is available here.

Stack

Based on #949 (fixture PR), which is based on CUJ7 #930. Retarget to main as the lower PRs merge.