Add bubblewrap path for running Nimi

[baileylu121]

Add Sandbox Feature Using Bubblewrap

This PR introduces a lightweight sandbox runner via mkBwrap that uses bubblewrap to run services in an isolated environment without requiring container runtimes like Docker or Podman.

The sandbox offers:

• Isolated filesystem with tmpfs mounts and selective read-only binds
• Environment variables and working directory configuration
• Namespace isolation (user, PID, UTS, IPC, cgroup)
• Read-only access to the Nix store for dependencies
• Writable tmpfs directories for runtime data

Added:

• Documentation in docs/sandbox.md with examples and comparison to containers
• Simple Redis example in examples/sandboxed.nix
• Test case to verify sandbox environment isolation
• New settings.bubblewrap module with comprehensive configuration options
• Implementation in nix/lib.nix that wraps the nimi binary in bubblewrap
• New public API to guarantee that the nimi module only gets evaluated once

The sandbox requires Linux with user namespace support and is marked as incompatible with Darwin systems.

---

Note

Medium Risk
Introduces a new mkBwrap execution path that changes how services can be launched (bubblewrap namespaces/mounts) and adds new module options/tests; misconfiguration could break runtime behavior on Linux or in CI.

Overview
Adds a new Bubblewrap-based sandbox runner (mkBwrap/mkBwrapWithConfig) that wraps the generated nimi runner in bwrap, with a new settings.bubblewrap module to configure mounts, env vars, working dir, and namespace/unshare flags (and marks the sandbox as unsupported on Darwin).

Refactors the Nix API to avoid double module evaluation by introducing mkNimiBinWithConfig and mkContainerImageWithConfig, and updates container defaults to set imageConfig.WorkingDir to /root.

Includes new docs (docs/sandbox.md + SUMMARY entry), a minimal sandboxed Redis example, a NixOS test validating env/chdir/tmpfs behavior, and updates flake.lock plus a small clap import cleanup in src/cli.rs.

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[baileylu121]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Add bubblewrap path for running Nimi #14 👈 (View in Graphite)
• Restrict flake systems to the big 4 #13
• master

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[github-actions]

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