Verify completed delegation before exiting the smart-routing CUJ

[lilly-luo]

CUJ4 opens /tasks as soon as the delegated child's answer appears, before requiring the parent to finish its reply. Claude 2.1.290 can open an individual agent's detail panel at that point; the test then times out waiting for a different task-list layout. This caused the subagent_only_v0 failure in #1083.

Make the test follow the delegation journey for both Claude and Codex: require the child's native answer, wait for the parent's completed native turn containing the delegated result, verify routing and served-model evidence after that turn, and require a normal /exit. The test no longer opens /tasks or parses its layout. Existing child-count, routing, and inference assertions remain.

Claude's native task-notification message belongs to the ongoing delegation turn, so the evidence reader now follows it to the parent's final answer. A real user message still ends the prior turn. Child-only answers, notification contents, and parent tool calls cannot satisfy parent completion.

Validation:

• Added a synchronous transcript regression covering child completion → native notification → parent still using tools → final parent result. It fails against the reader on main and passes with this change; no sleeps or TUI fixtures added.
• uv run --frozen pytest tests/test_cuj_evidence.py tests/test_e2e_cuj_helpers.py tests/test_integration_contract.py -q --disable-warnings: 275 passed.
• CI-equivalent Ruff check/format and ty checks passed. CUJ4 collects all 17 cases.
• Live agent/gateway CUJs were not run locally; CI validation is required.

Coverage READMEs describe the completed delegation-and-reply journey. The separate skill-toggle integration journey keeps its existing task-management UI check; CUJ4 no longer claims that coverage.