fix(nixos): fix prebuilt binaries via LD_LIBRARY_PATH + skip source build on read-only install

[jerome-benoit]

Summary

On NixOS (and other immutable-root systems), qmd embed crashes with EACCES because node-llama-cpp tries to compile llama.cpp into its read-only node_modules/ directory inside /nix/store. Even when source builds are skipped, the prebuilt binaries fail because they expect FHS paths like /lib64/libc.so.6 which don't exist on NixOS.

Two fixes:

1. LD_LIBRARY_PATH in wrapper — the flake wrapper sets LD_LIBRARY_PATH to include Nix's glibc and libstdc++, covering what the prebuilt binaries need without modifying them
2. skip source builds when the install directory is read-only (canWriteLlamaDir())

Changes

• flake.nix: Add glibc and libstdc++ to LD_LIBRARY_PATH in the wrapper. No binary modification needed — LD_LIBRARY_PATH takes precedence over RUNPATH for dlopen.
• src/llm.ts: Add canWriteLlamaDir() that probes write access on node-llama-cpp's llama/ directory. When unwritable, ensureLlama() passes build: "never" to getLlama().
• CHANGELOG.md: Unreleased entry under Fixes.

Closes #87

Testing

• nix build succeeds
• ./result/bin/qmd status shows CPU backend loaded, no EACCES, no NoBinaryFoundError
• ./result/bin/qmd embed completes successfully

[Copilot]

Pull request overview

This PR addresses NixOS/immutable-root failures when node-llama-cpp attempts to write compiled llama.cpp artifacts into a read-only node_modules path (e.g. under /nix/store) by redirecting build output to a user-writable cache directory.

Changes:

• Add resolveLlamaBuildDir() (+ QMD_LLAMA_BUILD_DIR) to compute a user-writable llama build directory following XDG cache conventions.
• Patch node-llama-cpp/dist/config.js during the Nix build to honor NODE_LLAMA_CPP_LOCAL_BUILDS_DIR, and set that env var in the Nix wrapper before JS starts.
• Document the fix in CHANGELOG.md and ignore generated node-llama-cpp-*.tgz artifacts.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/llm.ts	Adds a helper + default constant for resolving a writable llama build directory (XDG-based, env-overridable).
flake.nix	Applies a build-time patch to node-llama-cpp config and sets NODE_LLAMA_CPP_LOCAL_BUILDS_DIR in the wrapper.
CHANGELOG.md	Adds an Unreleased “Fixes” entry describing the NixOS redirect behavior and env vars.
.gitignore	Ignores node-llama-cpp-*.tgz artifacts.

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