all: add flake.nix setup and `CLAUDE.md` documentation (#6106) #10110

Job	Run time
Run unit tests	9m 0s
Subgraph Runner integration tests	3m 57s
Run integration tests	9m 6s
Clippy linting	1m 39s
Check rustfmt style	10s
Build in release mode	1m 1s
	24m 53s

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