git+file://'s dirty-tree fetcher reads tracked + staged content
from the index (not the working tree, not untracked files). so
staging is enough to make a new flake.nix or flake.lock visible
to nix without committing first. sync_agents now stages flake
.nix, runs lock, stages the resulting flake.lock, then commits
both together in a single 'regenerate meta flake' (or 'seed
meta from N agents') commit — no more two-commit churn.
prepare_deploy applies the same trick to the two-phase deploy:
runs nix flake update, stages flake.lock so nixos-container
update sees it, doesn't commit yet. finalize_deploy commits
with the deployed/<id> message on build success; abort_deploy
git-restores the staged lock back to HEAD on failure. meta
history continues to record only successful deploys (and now
one commit per success instead of one + amend).
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