[Unit PRs] Expose capability-gated strategy settings and operating guidance #566

Part of #553. Suggested step 12 of 13; one focused PR into main.

Outcome: Administrators and project users can understand and select the new strategy only where the completed capability is available, with its incremental-delivery tradeoffs explained.

Depends on: #563, #564, #565

Scope

• Add the option to existing platform/project settings and frontend service types using the admission contract from the first child.
• Keep it unavailable while the release capability is disabled or the selected provider/repository/workflow is unsupported; direct API validation must agree with the UI.
• Show that units merge independently, dependents wait for actual delivery, there is no final rollup, and cancellation/rewind cannot undo earlier merges.
• Update existing execution/settings/recovery documentation with the supported matrix, preparation requirements, rollback limits and rollout order. Keep defaults and existing in-flight settings unchanged.

Acceptance criteria

• Platform default/project override selection resolves consistently and is snapshotted only for new executions.
• Disabled and unsupported configurations cannot be selected or started through API bypass; the reason is visible.
• Changing project settings has no effect on an existing execution's strategy or target.
• Documentation explains individually safe units, feature flags where appropriate, review cadence and recovery after partial delivery.
• Focused settings/API tests, frontend build and documentation checks pass; the release capability remains off after this PR.

PR boundary and rollout

Settings and operating guidance for the implemented matrix. No deployment, default switch, provider expansion, new approval policy or remaining backend feature implementation.

Starting points

frontend/src/components/admin/PrStrategyCard.tsx, frontend/src/components/project-settings/GeneralTab.tsx, frontend/src/services/agents.ts, frontend/src/services/projects.ts, lambda/shared/pr-strategy.js, docs/concepts/execution.md.

Planning baseline: main at b5d68d5d2f12292d56a21aab917e4d8f9538aa0b (2026-10-15). Recheck current main and applicable guidance before implementation. Existing strategies remain supported; the new capability stays disabled until qualification.