docs: narrow RFC-0090 to project-author regression tooling

[meaningforger]

Summary

• Narrow RFC-0090 to project-author developer tooling: retain the shipped compile and metric-result checks, with a thin CLI over existing services.
• Remove comparison/attribution snapshots, host-managed policy suites, fixture orchestration, and baseline automation from the required roadmap. Explicitly distinguish removed scope from implemented capabilities.
• Align README, public CLI positioning, source-authoring guidance, testing ownership, and the shipped suite contract.
• Document a business regression (accidental SUM-to-AVG change) and its reviewed fix without automatically replacing expected values.

Compatibility and scope

Documentation only. No shipped command, suite schema, report format, query service, analytics service, or benchmark behavior changes. The uncommitted analytics implementation draft was withdrawn before this PR. The RFC scope revision remains subject to review; unimplemented extensions are not presented as completed.

Validation

• make docs-check — passed (143 Markdown files)
• git diff --check — passed
• Verified that this PR changes only Markdown documentation; no runtime tests required for this scope-only revision.