refactor(codeagent-wrapper): remove sisyphus agent and unused code

- Remove sisyphus agent from default config (references deleted sisyphus.md)
- Clean up unused variables: useASCIIMode, jsonMarshal
- Remove unused type: codexHeader
- Remove unused functions: extractMessageSummary, extractKeyOutput, extractTaskBlock
- Update tests to reflect 6 default agents instead of 7

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Co-Authored-By: SWE-Agent.ai <noreply@swe-agent.ai>

codeagent-wrapper/parser.go

@@ -59,14 +59,6 @@ const (
 	jsonLinePreviewBytes = 256
 )
 
-type codexHeader struct {
-	Type     string `json:"type"`
-	ThreadID string `json:"thread_id,omitempty"`
-	Item     *struct {
-		Type string `json:"type"`
-	} `json:"item,omitempty"`
-}
-
 // UnifiedEvent combines all backend event formats into a single structure
 // to avoid multiple JSON unmarshal operations per event
 type UnifiedEvent struct {