docs: establish staged feature planning workflow

docs/roadmap/features/template.md

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+# Feature <ID>: <Title>
+
+Status: proposed
+
+## Summary
+
+State the selected unit of work and the outcome it enables.
+
+## Problem or Opportunity
+
+Describe why this feature is needed and the current limitation it addresses.
+
+## Scope
+
+- List behavior and deliverables included in this feature.
+
+## Non-goals
+
+- List adjacent behavior intentionally excluded from this feature.
+
+## Target End State
+
+Describe the observable repository, deployment, and operator state after the
+feature is complete. Keep this independent of the implementation sequence.
+
+## Repository Ownership
+
+Identify every repository involved and the durable responsibility each one
+owns. Distinguish repositories that will change from read-only sources of
+application contracts.
+
+## Constraints and Settled Decisions
+
+Link the MVP roadmap and applicable decision records. Record technical or
+operational constraints that implementations must preserve.
+
+## Rationale and Alternatives
+
+Explain material tradeoffs and why the selected end state is preferred. Omit
+this section when the choice is mechanical and needs no durable rationale.
+
+## Acceptance Criteria
+
+- Define observable, testable conditions required for feature completion.
+
+## Dependencies and Follow-up Work
+
+List prerequisites and explicitly deferred work without expanding this
+feature's scope.