Set up dev environment + AGENTS.md for NexusOS

[Aidiotic]

Summary

Sets up and verifies the NexusOS development environment (a Bash-based custom Debian distro builder) and documents durable, non-obvious dev/run guidance for future Cursor Cloud agents.

• Adds AGENTS.md with a ## Cursor Cloud specific instructions section (lint, build, in-distro CLI gotchas, and where the real product code lives).
• No product code changes.

Environment

• Installed system deps (baked into VM snapshot): shellcheck, debootstrap, xz-utils (python3/make/git already present).
• Startup update script is minimal + network-free: ensures shell scripts are executable (there are no package-manager dependencies to refresh).

Verification

• ✅ make validate — healthcheck passes (shell syntax, JSON configs, os-release, platform detect, version, permissions).
• ✅ shellcheck -e SC1091,SC2034,SC2164 across all scripts — only pre-existing advisory SC2005/SC2295 notices (non-blocking, matches CI).
• ✅ sudo NEXUSOS_CI_MINIMAL=1 ./build/rootfs/build-x86_64.sh — produced a real 78M Debian bookworm (12.14) rootfs releases/nexusos-x86_64-rootfs.tar.xz rebranded as NexusOS 1.0.
• ✅ nexus-store list — software center lists all 8 catalog packages.
• ✅ ./scripts/detect-platform.sh → linux x86_64 false.

Verification log:
nexusos_verification.log