feat: use musl targets and add SHA256 checksums to release workflow

[Copilot]

The release workflow used linux-gnu targets instead of the specified linux-musl targets, and had no checksum generation or publication.

Changes

Cross.toml

• Updated default-target from x86_64-unknown-linux-gnu → x86_64-unknown-linux-musl

.github/workflows/ci.yml — release-binaries job

• Switched both Linux targets to musl:
  • x86_64-unknown-linux-gnu → x86_64-unknown-linux-musl
  • aarch64-unknown-linux-gnu → aarch64-unknown-linux-musl
• Added SHA256 checksum generation after each binary is produced, portable across Linux and macOS runners:

  - name: Generate SHA256 checksum
    shell: bash
    run: |
      if command -v sha256sum &>/dev/null; then
        sha256sum "${{ matrix.artifact_name }}" > "${{ matrix.artifact_name }}.sha256"
      else
        shasum -a 256 "${{ matrix.artifact_name }}" > "${{ matrix.artifact_name }}.sha256"
      fi

• Updated artifact and release upload steps to include the .sha256 file alongside each binary

[devactivity-app]

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