build: add nix flake packaging

[ReStranger]

What / Why / How

Add Nix flake packaging for context-mode so the project can be built and run as a Nix package instead of only via npm-style installation. This adds a minimal flake-based package that wraps the bundled CLI with nodejs_22, includes the runtime assets the CLI expects next to itself, enables allowUnfree in the flake-local Nixpkgs import for the project's Elastic-2.0 license, and exposes the package as both packages.default and packages.context-mode plus an apps.default entry for nix run.

Affected platforms

• Claude Code
• Cursor
• VS Code Copilot (GitHub Copilot)
• Gemini CLI
• OpenCode
• KiloCode
• Codex CLI
• OpenClaw (Pi Agent)
• Kiro
• Antigravity
• Zed
• All platforms

Test plan

• Build the package with nix build .#context-mode
• Run ./result/bin/context-mode --help
• Run ./result/bin/context-mode doctor
• Run nix run .# -- --help
• Verify bundled runtime detection includes node, tsx, and python3 from the wrapped PATH

Checklist

• Tests added/updated (TDD: red → green)
• npm test passes
• npm run typecheck passes
• Docs updated if needed (README, platform-support.md)
• No Windows path regressions (forward slashes only)
• Targets next branch (unless hotfix)

Cross-platform notes

Our CI runs on Ubuntu, macOS, and Windows.

• This change does not modify application path handling logic; it only adds Nix packaging metadata and a wrapper script
• The package uses the prebuilt cli.bundle.mjs and installs runtime assets alongside it, matching the existing relative-path expectations in the CLI
• nodejs_22 is used so Linux builds can rely on the built-in SQLite path already supported by the project
• allowUnfree is configured in the flake's local nixpkgs import, so users do not need --impure or external Nix config to build the package