Migrate repository automation to xtask

[P4suta]

Summary

• add a Rust 1.75-compatible cargo xtask workspace member for repository checks, release artifacts, SBOM validation, tag verification, and contained mutation testing
• keep existing just commands as thin wrappers and move CI/release workflow logic out of PowerShell
• remove all tracked .ps1 files, pin typos 1.48.0 in CI, and simplify README, rustdoc, ADRs, and project policy text

Why

PowerShell accounted for a disproportionate share of the repository and duplicated behavior across local and GitHub workflows. The mutation runner also embedded C# and custom Win32 FFI. Centralizing automation in Rust makes the behavior testable, keeps the MSRV explicit, and reuses windows-spawn's own DropPolicy::KillTree containment.

The CycloneDX path uses the expanded windows-spawn package manifest so the release SBOM excludes the non-published xtask workspace member.

Impact

The public windows-spawn API, MSRV, runtime behavior, and public-api snapshot are unchanged. Existing just recipe names remain available, and developers can also call cargo xtask ... directly.

Validation

• cargo xtask fmt
• cargo xtask clippy
• cargo xtask test
• cargo xtask doc
• cargo xtask msrv
• cargo xtask cross-targets
• cargo xtask linux-empty
• cargo xtask public-api
• cargo xtask supply-chain
• cargo xtask reuse
• cargo xtask typos
• cargo xtask coverage
• cargo xtask package-check --allow-dirty
• cargo xtask release-candidate --allow-dirty
• cargo xtask mutants --output target/mutants/smoke -- --list
• actionlint
• git diff --check
• gitleaks git . --redact --no-banner
• gitleaks dir . --redact --no-banner