feat(maestro): add a nix flake and Home Manager module

Ships `maestro`, `maestro-web` and the SPA bundle as flake packages, plus a
`services.maestro` Home Manager module that runs the daemon and the web UI as
systemd user services.

`maestro-web`'s build.rs now honours MAESTRO_WEB_DIST and hands the resolved
bundle path to `include_dir!` through MAESTRO_WEB_BUNDLE_DIR. Without it the
Nix build would have to run `pnpm install` mid-compile, which the sandbox has
no network for. Unset, the pnpm path is unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -57,6 +57,8 @@ crates/
 ├── maestro-installer/   # `maestro-install` wizard: cargo install + systemd (NAP-225)
 └── maestro-e2e/         # cross-process integration tests (NAP-217)
 web/                     # browser SPA (NAP-218)
+flake.nix                # Nix flake: `maestro`, `maestro-web`, `webFrontend`
+nix/                     # Home Manager module (`services.maestro`)
 ```
 
 See `docs/INSTALL.md` for end-user install instructions.