docs: Settings Phase 2 feature-folder migration plan

[antosubash]

Summary

Implementation plan for Phase 2 of the vertical-slice feature-folder migration: converting the Settings module (16 API endpoints across 3 resource groups, 2 cross-module contract services) to the layout validated by the Notifications pilot in #200.

Docs only — no code changes. Execution is gated on #200 merging (the plan depends on the migration tooling and conventions shipped there).

What the plan covers

• 8 implementation tasks, modeled on the Phase 1 plan with TDD-style steps, exact file paths, exact code blocks, and per-task commit messages.
• Two services to partial-class-split: `SettingsService` → `ISettingsContracts` (6 methods) and `PublicMenuService` → `IPublicMenuProvider` (9 methods).
• 25-row TSV manifest scoped: 5 Infrastructure moves, 16 endpoint moves, 4 feature-scoped Contracts DTO moves. Entities, value objects, shared DTOs, events, and the contract interface stay at `Contracts/` root.
• Recon-confirmed: zero cross-module blast radius for the Contracts split. The 11 external consumers of `SimpleModule.Settings.Contracts` all reference root-namespace types only (`ISettingsContracts`, `Setting`, `SettingsFilter`, events).
• Integration tests deliberately stay in `Integration/` — they cover cross-feature HTTP flows by design (lesson learned in refactor: vertical-slice feature folders (Notifications pilot) #200).
• Explicit execution prerequisites and exit criteria. Cross-module consumer build check baked into pilot verification.

Why a separate PR

• Docs-only: lower review weight than refactor: vertical-slice feature folders (Notifications pilot) #200's implementation work.
• Won't interfere with refactor: vertical-slice feature folders (Notifications pilot) #200 review or rebases.
• After refactor: vertical-slice feature folders (Notifications pilot) #200 lands and any spec/script changes are folded in, this plan can be executed from a fresh branch off updated `main`.

Test plan

• Review reads end-to-end — the plan should be runnable as-written by an agent with no prior context.
• Recon claims in the plan match the actual codebase (file counts, service method signatures, cross-module consumer locations).
• No requirement in the plan depends on something not in refactor: vertical-slice feature folders (Notifications pilot) #200 or on `main`.

Related

• Depends on refactor: vertical-slice feature folders (Notifications pilot) #200 (Notifications pilot) for the migration tooling, spec conventions, and Phase 1 lessons it references.
• Design spec: `docs/superpowers/specs/2026-05-14-vertical-slice-feature-folders-design.md` (merged via refactor: vertical-slice feature folders (Notifications pilot) #200).

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