feat!: replace target integration with host providers and release workflows

[Morbden]

Summary

• Replaced the legacy galfus-target integration with galfus-host and optional, execution-scoped I/O providers.
• Kept Workspace as the isolated execution facade: concrete providers are supplied only at runtime, and missing providers fail only when the related builtin executes.
• Added native CLI and buffered playground providers, configurable std/io::read terminators, CLI exit-code propagation, and expanded showcase coverage.
• Removed galfus-target and its exclusive dependencies.
• Added versioned VS Code and playground WebAssembly artifact workflows.
• Consolidated repository automation under scripts/src/cmd.ts with Commander commands, including github set-version.
• Improved generic constraint resolution, generic inference/specialization, workspace loading, and regression coverage.

Linked Issues

• None.

Change Type

• changelog:breaking
• changelog:feature
• changelog:fix
• changelog:refactor
• changelog:performance
• changelog:docs
• changelog:internal (exclude from public changelog)

User-facing Changelog Note

• Embedders now provide optional host I/O providers when running a workspace; the legacy target integration crate has been removed.
• Release branches can build versioned VS Code packages and browser-ready playground WebAssembly artifacts.

Risks / Rollback

• Main risks:
  • Workspace::run and removal of galfus-target are breaking changes for embedders.
  • Host-backed builtins require a matching provider at runtime.
  • Web artifact generation depends on the pinned wasm-bindgen-cli version.
• Rollback plan:
  • Revert this PR to restore the previous target capability integration and automation workflows.