Enable dependabot updates for flake.lock

[BradLugo]

Dependabot supports the nix ecosystem since April 2026; this keeps the
flake.lock inputs from going stale. It only covers flake.lock, not the
vendorHash in flake.nix, which the autofix workflow handles.

This PR is part of a stack containing 5 PRs:

1. main
2. Fix nix flake build #247
3. Add nix flake build check to CI #248
4. Document nix flake install in README #249
5. Auto-fix stale nix vendorHash on dependabot PRs #250
6. "Enable dependabot updates for flake.lock" (this PR)

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