chore(agents): improve ca-test-infra-improver — strengthen duplicate avoidance

Approved proposal: #2370
Pattern: Duplicate work / issue tracker noise
Evidence: Workers filed ~30 duplicate issues across 8 topic clusters
(e.g., 7 issues for "parallelize E2E tests", 5 for "increase default
parallelism"). Current duplicate avoidance only checks title prefix.
Fix: Add keyword-based search, explicit similarity definition, and
cross-worker coordination with a conservative "skip when uncertain"
approach matching the UAT tester's proven pattern.

ISSUES CLOSED: #2370

.opencode/agents/ca-test-infra-improver.md

@@ -443,8 +443,8 @@ Before filing ANY issue, you MUST perform rigorous duplicate checking:
 5. **If ANY existing issue proposes the same or substantially similar
    improvement**: **SKIP** — do not file. It is far better to miss one
    improvement suggestion than to file the 7th duplicate of the same idea.
-   Previous sessions created 48+ TEST-INFRA issues with significant overlap
-   across 8 topic clusters.
+    Previous sessions created 48+ TEST-INFRA issues with significant overlap
+    across 8 topic clusters.
 
 6. **Post-filing verification**: After filing an issue, wait 5 seconds and
    re-check for duplicates (another parallel worker may have filed the same