Refactor install.sh: split into modules to stay under 600-line limit

[nichm]

Problem

install.sh has grown to 768 lines after adding AI-dev tools in #13, exceeding the 600-line limit enforced by the pre-commit file-length check.

Solution

Split install_deps() into per-platform modules (e.g. lib/setup/deps-brew.sh, deps-apt.sh, deps-rpm.sh, etc.) and source them from install.sh. Each module stays under 200 lines.

Alternatively, extract the helper functions (_sc_brew, _sc_npm, _sc_cargo, _sc_uv) into lib/setup/install-helpers.sh.

Impact

• Improves maintainability and testability per-platform
• Aligns with the existing lib/ module structure

Technical debt introduced by: #13

[github-actions]

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