chore: drop darwin from flake (Linux-only)

[systemfsoftware-maker]

Removes macOS from the fork's build and CI config, following the org rule that systemfsoftware repos have no macOS. Base is main at c93d6c8, which includes #3.

Changes

flake.nix

• flake-utils.lib.eachDefaultSystem is replaced with eachSystem [ "x86_64-linux" "aarch64-linux" ].
• Removed the aarch64-darwin and x86_64-darwin node-modules hashes. x86_64-linux keeps its hash, and aarch64-linux stays on fakeHash as before.
• Removed the optionals isDarwin [ pkgs.darwin.cctools ] entry from nativeBuildInputs.
• In the wrapper, removed --set DYLD_LIBRARY_PATH, the Darwin/Metal residency comment, and the uname -s = Darwin branch of --run. The MCP log-level part of --run is unchanged. LD_LIBRARY_PATH no longer depends on isLinux; on Linux the value is the same as before.
• meta.platforms changed from platforms.unix to platforms.linux.

Other config

• .github/workflows/ci.yml: removed DYLD_LIBRARY_PATH: /opt/homebrew/opt/sqlite/lib from the bun test step.
• scripts/test-all.mjs: removed the darwin-only GGML_METAL_NO_RESIDENCY env injection.
• src/test-preload.ts: updated the header comment so it lists only the injector that is left (bin/qmd).

Verification (local, x86_64-linux)

$ nix build . -L                 → exit 0
$ ./result/bin/qmd --version     → qmd 2.8.3 (exit 0)
$ nix flake check -L             → all checks passed! (exit 0; aarch64-linux omitted as incompatible host)

The generated wrapper exports only LD_LIBRARY_PATH (sqlite, glibc, gcc-lib) plus the MCP log-level block.

macOS handling left in place (runtime source and docs, not changed here)

• bin/qmd: the darwin GGML_METAL_NO_RESIDENCY launcher block.
• src/db.ts: Homebrew libsqlite3.dylib setCustomSQLite lookup, plus the macOS hint in the sqlite-vec error.
• src/llm.ts: isDarwinMetalMitigationActive, installDarwinExitGuard, isDarwinExitGuardInstalled, and the Metal dispose comments.
• src/cli/qmd.ts: doctor's "darwin metal residency" check, the metal (macOS Metal backend) label, and the QMD_METAL_KEEP_RESIDENCY/GGML_METAL_NO_RESIDENCY env help.
• src/store.ts: the Homebrew SQLite / BREW_PREFIX error text.
• src/cli/mcp-pid.ts: the ps fallback comment for macOS.
• package.json optionalDependencies: sqlite-vec-darwin-arm64 and sqlite-vec-darwin-x64. These are runtime native packages; removing them changes the lockfiles and the published package.
• scripts/repro-metal-rsets-crash.mjs: a macOS-only repro script.
• README.md: the macOS Homebrew SQLite requirement.
• Tests covering the runtime paths above (test/bin-wrapper.test.ts and others).
• flake.nix devShell: BREW_PREFIX export. It is a Homebrew name but not macOS-specific, and nothing in src/ reads it.
• Lockfiles: transitive darwin platform packages from upstream dependencies such as esbuild, oxlint and typescript.