feat(setup): platform-aware wizard — platform selection, smart workflow pre-seeding, macOS-only simulator/device skip

[ichoosetoaccept]

The xcodebuildmcp setup wizard has no awareness of which Apple platforms a project targets. As a result:

• Workflow selection is a flat list with no smart defaults — users don't know which workflows are relevant for their platform.
• The simulator prompt appears even for macOS-only projects that don't need one.
• There's no platform written to sessionDefaults, so the agent has to guess the platform on every command.

Proposed changes

Platform selection step — a multi-select prompt asking which platforms the project targets (macOS, iOS, tvOS, watchOS, visionOS). Previous choices are pre-loaded on re-run, inferred from sessionDefaults.platform, sessionDefaults.simulatorPlatform, or the existing workflow set.

Smart workflow pre-selection — based on the selected platforms, the wizard pre-seeds the workflow selection with the appropriate set (e.g. macOS-only gets the macOS + common workflows; iOS gets the simulator workflow set). Users can override freely.

Skip simulator and device prompts for macOS-only — when macOS is the only selected platform, both the simulator and physical device selection prompts are skipped entirely.

platform written to sessionDefaults — for single-platform selections, platform is persisted to config.yaml. For multi-platform projects it is omitted so the agent can choose per-command. On macOS-only, stale simulatorId/simulatorName/deviceId are cleaned up from the config.

--format mcp-json includes XCODEBUILDMCP_PLATFORM — when a single platform is selected, the ready-to-paste MCP config JSON block includes XCODEBUILDMCP_PLATFORM.

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Note: #269 originally tracked this feature and was closed pointing to #272, but the platform wizard described there was not part of that PR. This issue picks up the remaining work.