fix(ci): drop pkgs.chromium from default devShell

[thedavidmeister]

Summary

• Remove pkgs.chromium from the default Linux devShell. cache.nixos.org does not have a prebuilt chromium for the pinned nixpkgs revision, so Linux runners spend hours compiling from source and hit the 6h job timeout.
• Drop the chromium.test.bats smoke test that asserted chromium was on PATH; nothing in rainix actually invokes it.
• README + slim-shell comments updated to drop the now-stale "no chromium" enumeration.
• The sol-shell / rust-shell closure tests continue to assert chromium is absent from those slim closures.

Closes #170.

Test plan

• CI green on Linux without timing out
• sol-shell closure test (asserts no chromium) still passes
• rust-shell closure test (asserts no chromium) still passes

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 617ddf56-6731-4e34-a2ab-9a1245df78c3

📥 Commits

Reviewing files that changed from the base of the PR and between 49fb732 and 37c992d.

📒 Files selected for processing (3)

• README.md
• flake.nix
• test/bats/devshell/default/chromium.test.bats

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-05-13-drop-chromium

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