rules: separate focused local checks from hosted validation

[shinaoka]

Summary

• define incremental non-release builds as the default for local edit-test loops
• keep local PR checks proportional to code, documentation-only, and CI-only changes
• assign comprehensive clean workspace, coverage, backend, docs, and hardware validation to hosted CI
• reserve release mode for performance and optimization-sensitive validation

Verification

• validated every relative Markdown link from rules/index.md
• git diff --check
• confirmed the PR changes only rules/common/docs-and-tests.md

This is a documentation-only shared-rule update. Repository-local policies remain authoritative when a project needs stricter validation.