[codex] Add conda-forge Intel Mac dev env

[anth-volk]

Fixes #218

Summary

• Add ./scripts/install.sh with explicit production, development, and Intel macOS development install modes.
• Add a conda-forge-backed Intel macOS developer environment manifest under envs/ with Python 3.13 and PyTorch 2.11.
• Remove the uv lock/package metadata workaround that omitted torch on Intel macOS, and document that Intel macOS is development/testing-only.
• Add tests covering install script behavior, Intel macOS rejection for prod/dev paths, and the conda-forge environment metadata.

Validation

• bash -n scripts/install.sh
• shellcheck scripts/install.sh
• uv lock --check
• UV_CACHE_DIR=/tmp/microplex-us-uv-cache uv run --no-sync pytest -q tests/test_install_script.py tests/test_project_metadata.py -> 9 passed
• CONDA_EXE=$HOME/miniforge3/bin/conda MICROPLEX_US_INSTALL_UNAME_S=Darwin MICROPLEX_US_INSTALL_UNAME_M=x86_64 ./scripts/install.sh --dev-intel-mac -> microplex-us Intel dev env ready: x86_64 torch 2.11.0
• /Users/administrator/miniforge3/envs/microplex-us-intel/bin/python -m pytest -q tests/test_install_script.py tests/test_project_metadata.py -> 9 passed
• git diff --check

Notes

• GitNexus checks are currently blocked in this worktree by Cannot destructure property 'package' of 'node.target' as it is null; I used explicit scoped staging plus the validation above as the fallback.