fix: guard Homebrew installs on Linux

[steipete]

Fixes #147.

What changes

• rewrite generated formulas into a canonical macOS-only shape
• add an explicit Linux odie message that points users to npm
• add regression coverage for the guarded formula layout
• clarify release/docs language around Homebrew being macOS-only today

Verification

• pnpm vitest run tests/release.formula.test.ts
• pnpm -s check

[steipete]

Landed via temp rebase onto main.

• Gate: pnpm vitest run tests/release.formula.test.ts and pnpm -s check
• Land commit: c8e4183785f76bcbe457956fa8cc78d42e81b280
• Merge commit: cdaa99b34f2dabe591938bf457a4d113bf4d3562

Thanks @steipete!

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Count non-do Ruby blocks when skipping platform sections

skipDoBlock increases depth only for lines ending in do but decreases depth for every end, so an on_macos/on_linux block containing if/unless/case can terminate early at an inner end. In that case stripExistingPlatformConfig leaves the outer end in place, and updateFormulaForMacArtifacts writes malformed Ruby with an extra end, breaking formula generation for that release.

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