Add nix flake

[lanice]

Adds flake.nix, together with some README + AGENTS + CI bits. The flake allows an additional method to install/use dbosctl, e.g. via nix run github:dbos-inc/dbos-ctl.

Added a step to build the nix variants in CI.

[devhawk]

Nice — the flake is clean and the nix flake check --all-systems note about Nix 2.19 is a good catch. Four comments inline: one I'd want addressed before merge (missing x86_64-darwin), three minor.

Also verified as fine, for the record: env.CGO_ENABLED = 0 composes correctly with buildGoModule (current module.nix merges via args.env.CGO_ENABLED or go.CGO_ENABLED); --all-systems only evaluates the non-native checks rather than building them; self.rev or self.dirtyRev or null associates as intended and the optionalString/optional guards keep isoDate lazy; dbosctl completion {bash,zsh,fish} in postInstall works since rootCmd doesn't disable Cobra's default completion command; and go_1_25 matches go.mod with no toolchain directive for GOTOOLCHAIN=local to trip over.