feat: add linear GitHub Actions CI

[zatevakhin]

Summary

• Add a single linear CI graph: format → clippy → test → build → nix.
• Reuse qmtui toolchain/cache/Nix installer actions and vllm.cpp-rs concurrency/timeouts, without parallel fan-in or cross-compile/release jobs.
• Pin flake.lock so the nix job can run alejandra --check and nix develop --command cargo test.
• Build the release binary and smoke qmtacp --help. Live ACP/server tests stay out of CI.

Stacked on #3 (v2/agent-control).

@coderabbitai ignore

Why

qmtacp had no CI. PRs currently rely on local cargo test / clippy, so format, lint, tests, and the flake shell can drift independently. A linear needs chain fails fast on style, then lints, then unit tests, then a real binary, then Nix, without diamonds or extra workflows.

Test plan

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-targets --all-features (26 passed)
• cargo build --release --bin qmtacp and ./target/release/qmtacp --help
• alejandra --check flake.nix
• GitHub Actions run on this PR

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