docs(rfc): distinguish lifecycle from delivery maturity - #5348

Goal and result

The RFC index described the accepted Shared Goal Alignment RFC as still a draft. Replace that lifecycle claim with its actual delivery boundary: Stage 1/2 foundations implemented, Stage 3+ unshipped.

Clarify in the authoring template that Delivery maturity: Proposal is compatible with an Accepted design. Keep lifecycle in the canonical header/generated index and implementation facts in delivery prose. No broad prose denylist or new status registry is added.

Scope and review

• Documentation only: two existing files, seven added lines and one removed line. No runtime, capability, provider, default, permission or public first-viewport change.
• Bounded maintenance pass: strengthened the existing RFC authoring contract; no additional abstraction or tests are needed for this wording correction.
• All candidate paths are core documentation. Generated dependency state is excluded. No private state, raw logs, local paths or credentials are included.
• Separate from the quota action-authority RFC and docs: reconcile merged qualification and contributor checkpoints #5235's delivery-checkpoint reconciliation. No merge requested.

Validation

Tested content is committed at 0021219b3 (finished; static/public fixtures).

An initial isolated scan lacked its registry; supplying an empty disposable registry resolved that setup error. Runtime/live qualification is not applicable to this docs-only change. Commit is DCO signed; PR base is main, with merge base b79bcb194.