feat: expose qmtacp as the default flake package

[zatevakhin]

Summary

• Add packages.qmtacp / apps.qmtacp and set both as default.
• nix run github:querymt/qmtacp and nix profile add github:querymt/qmtacp now install/run the CLI.
• CI nix job also runs nix build.

Stacked on #5 (v4/release-ci).

@coderabbitai ignore

Why

The flake was a rust-overlay devShell only, so nix run / nix profile add failed. qmtui already exposes a default package; this matches that so the CLI can be run without cloning.

Test plan

• alejandra flake.nix
• nix build .#qmtacp and ./result/bin/qmtacp --help
• nix run . -- --help
• nix run .#qmtacp -- --help
• GitHub Actions nix build on this PR

[coderabbitai]

Important

Draft PR not reviewed

Draft PRs are not automatically reviewed by default.

• Trigger a manual review

To automatically review draft PRs, update your CodeRabbit configuration:

reviews:
  auto_review:
    drafts: true

---

Note

🎁 Summarized by CodeRabbit Free

Your organization is on the Free plan. CodeRabbit will generate a high-level summary and a walkthrough for each pull request. For a comprehensive line-by-line review, please upgrade your subscription to CodeRabbit Essentials by visiting https://app.coderabbit.ai/settings/billing.

_{Comment @coderabbitai help to get the list of available commands.}