fix(agent): polish interrupt-resume flow for merge readiness

DELETION_LOG.md

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+# Deletion Log
+
+## 2026-03-03 feature polish
+
+- Scope: `backend/src/v1/agent/*` and `backend/tests/unit/v1/agent/*` only.
+- Candidate review source: scoped `refactor-cleaner` run for the directories above.
+
+### Executed cleanup
+
+1. Merged duplicated newline validation logic in `backend/src/v1/agent/service.py`.
+   - Before: duplicated checks in `prepare_resume`, `stream_run`, `stream_resume`.
+   - After: centralized `_validate_no_newlines` helper.
+   - Behavior impact: none (same validation semantics).
+
+2. Merged duplicated SSE event string formatting in `backend/src/v1/agent/service.py`.
+   - Before: repeated `f"data: {json.dumps(...)}\n\n"` fragments.
+   - After: centralized `_sse_data` helper.
+   - Behavior impact: none (same payload format).
+
+### Candidates not deleted (insufficient evidence)
+
+- `backend/src/v1/agent/crewai_flow.py`
+  - Reason: candidate report suggested possible dead code, but no deletion was done in this polish pass because cross-module usage certainty was insufficient.
+- Legacy `run()` path in `backend/src/v1/agent/service.py`
+  - Reason: potentially still relied on by non-scope code paths; deletion deferred.