docs(maintainers): define the feature proposal workflow (#7000)

The handbook says substantial features need Product Owner approval
before implementation, but not how a proposal gets there, what counts
as substantial, or what happens when no answer comes. Add the full
workflow: triage within 7 days, a decision brief, four product labels,
a lazy-consensus timeout that only applies to big-but-reversible work,
and definitions for every judgement call (small, big, reversible, new
direction, fits ROADMAP, objection, recorded decision).

The Feature Request form gets three optional fields for the brief, and
the PR template, CONTRIBUTING and triage point at the workflow.

docs/maintainers/triage.md

@@ -26,6 +26,8 @@ Record what is known and distinguish it from what remains unverified:
 - Priority, ownership, and relevant domain expertise.
 - Remaining unknowns, risks, and safe recovery or workaround when available.
 
+Feature requests follow the [feature proposal workflow](product-and-features.md#feature-proposal-workflow): triage within 7 days, and a product label on every triaged feature issue.
+
 Small and obvious fixes may go straight to a focused PR, consistent with [CONTRIBUTING.md](../../CONTRIBUTING.md) and [changes and review](changes-and-review.md).
 
 ## Practical priority language