chore(devenv): add Nix development shell

[elezar]

Summary

Add a checked-in Nix/devenv shell for OpenShell local development, including platform-specific build prerequisites for Linux and macOS. The branch also improves musl binary staging and local setup messaging so contributors get clearer behavior from the new shell.

Related Issue

None.

Changes

• Add devenv.nix, devenv.yaml, and devenv.lock for an optional Nix/devenv development shell.
• Configure Z3, pkg-config, libclang, e2fsprogs, Docker CLI, GitHub CLI, and VM build prerequisites through devenv.
• Add Zig-backed musl toolchain wrappers for static supervisor builds and binary staging.
• Improve shell entry messages for mise trust, missing tool installs, and open-file limits.
• Update contributor, build, and VM driver docs for the devenv workflow.
• Preserve Python wheel relinking before native maturin builds.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

No new tests were added; existing Python and Rust tests ran through mise run pre-commit.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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