Fix public Apple platform parity in MCP output schemas

[devin-ai-integration]

Problem

The MCP output schemas for boot and shutdown use the internal PLATFORMS
enum (apple | android | linux | web), while the public command contracts and
runtime emit PublicPlatform (ios | macos | android | linux | web).
prepare has the same schema mismatch, although it is not MCP-exposed.

The current boot schema test checks required keys only, so an ios fixture
passes without validating enum membership.

Scope

• Use the public platform vocabulary in public command output schemas.
• Validate representative structured content against the complete advertised
  schema, including enums/consts and nested required fields.
• Cover both Apple public leaves and preserve additive fields (cost, future
  fields); do not introduce additionalProperties: false.
• Keep internal platform identity as apple; do not change runtime wire values.

Owning files

• src/mcp/command-output-schemas.ts
• src/mcp/__tests__/command-tools.test.ts
• src/contracts/device.ts
• src/contracts/prepare.ts
• src/kernel/device.ts

Acceptance criteria

• Valid ios and macos boot/shutdown results satisfy their advertised MCP
  schemas.
• Internal apple is not advertised on these public result fields.
• Invalid enum values fail the schema validation test.
• prepare mirrors its PublicPlatform contract.
• pnpm check:affected --base origin/main --run passes.

Non-goals

• Public ios/macos to apple migration.
• Repository-wide JSON Schema generation or strict schemas.