chore(nix): add flake.lock at repo root + k9-svc/

Both flake.nix files exist and load upstream inputs (nixpkgs,
flake-utils) but the corresponding flake.lock files were untracked.
Committing the locked state ensures reproducible `nix develop`
invocations and satisfies the supply-chain-pinning posture the RSR
workflow set expects.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath