ci: add codecov configuration

Add codecov.yml with coverage targets and ignore patterns:
- Target coverage: 80% for project and patches
- Ignore test/bench/example directories
- Configured comment layout and CI wait behavior

Author: bug-ops

.github/codecov.yml

@@ -0,0 +1,37 @@
+codecov:
+  require_ci_to_pass: true
+  notify:
+    wait_for_ci: true
+
+coverage:
+  precision: 2
+  round: down
+  range: "70...100"
+  status:
+    project:
+      default:
+        target: 80%
+        threshold: 2%
+        informational: false
+    patch:
+      default:
+        target: 80%
+        threshold: 5%
+        informational: false
+
+comment:
+  layout: "reach,diff,flags,tree,footer"
+  behavior: default
+  require_changes: false
+  require_base: false
+  require_head: true
+
+ignore:
+  - "tests/"
+  - "benches/"
+  - "examples/"
+  - "crates/*/tests/"
+  - "crates/*/benches/"
+  - "crates/*/examples/"
+  - "crates/mcp-cli/"
+  - "crates/mcp-examples/"