Add lane-parallel ContextBench readiness workflow #1

Job	Run time
lane-readiness (codegraphcontext)	1m 8s
lane-readiness (raw-native)	48s
lane-readiness (codebase-memory-mcp)	1m 4s
lane-readiness (grepai)	1m 44s
lane-readiness (codebase-context)	11m 15s
	15m 59s

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