Clarify project guidance and release workflows

[SandroMaglione]

Summary

• make the project priorities explicit: type safety, opinionated semantics, readable models, and Effect-core alignment
• define the ownership and naming boundary between local machine capabilities and Effect Cluster
• document the pre-1.0 minor/patch policy and preference for a clear long-term API over compatibility aliases
• add explicit-only $effect-machine-pr and $effect-machine-release repository skills for repeatable PR and release lifecycles

Changeset

• Added or updated for a library or package-metadata change
• Not required because this PR does not change src/ or package.json

Validation

• pnpm check
• Relevant example checks, when examples changed
• Automated type-performance measurement passed or was not required
• Automated runtime- and memory-performance measurement passed or was not required

Both repository skills also pass Codex's quick_validate.py structural validator. pnpm perf:runtime passed locally as the full-lifecycle workflow smoke test; GitHub's classifier determines whether a base-versus-PR comparison is required for this documentation and tooling-only change.