fix(ci): add baseline CPU variant binaries for legacy hardware support

[code-yeongyu]

Problem

Users with CPUs lacking AVX2 instructions (for example, Intel Ivy Bridge) cannot use oh-my-opencode past v3.1.3 because only AVX2-targeted binaries are published and selected.

Root Cause

publish-platform.yml published only 7 platform packages, missing baseline x64 targets for non-AVX2 CPUs. Runtime package selection also had no baseline fallback strategy.

Fix

• Expand platform publish matrices to include baseline variants: darwin-x64-baseline, linux-x64-baseline, linux-x64-musl-baseline, windows-x64-baseline
• Update runtime package resolution to consider baseline fallback packages and retry with baseline when the primary binary exits with SIGILL
• Add optional baseline platform dependencies and shared type declarations for platform helper exports
• Update postinstall binary check to validate candidate packages in fallback order

Closes #2121

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Summary by cubic

Adds baseline x64 binaries and fallback logic so oh-my-opencode runs on CPUs without AVX2. Expands publishing and install checks to prefer baseline when AVX2 is missing; closes #2121.

• New Features
  • Publish baseline variants: darwin-x64-baseline, linux-x64-baseline, linux-x64-musl-baseline, windows-x64-baseline; workflows build/publish them and package.json lists them as optionalDependencies.
  • Runtime: detect AVX2 (or OH_MY_OPENCODE_FORCE_BASELINE), resolve candidate packages in order, retry on SIGILL, and preserve correct signal exit codes.
  • Postinstall: verify candidates in order and report the resolved package with clear errors when none are installed; added getPlatformPackageCandidates and type defs.

^{Written for commit cc5e9d1. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 8 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Requires human review: Increases installation size for all x64 users by downloading multiple variants, and refactors core execution logic with potential side effects during SIGILL retries.}