Platform support matrix in CLI/MCP: mark platform-specific commands

[rafiki270]

Problem

Commands that only work on specific platforms are not marked as such in CLI help or MCP tool descriptions. LLMs discover platform limitations only after hitting PLATFORM_NOT_SUPPORTED errors.

Platform-specific commands with no marking:

Command	iOS	Android	macOS
set-renderer / get-renderer	-	-	macOS only
safari-auth	iOS only	-	-
debug-screens	iOS only	-	-
set/get-debug-overlay	iOS only	-	-
set/get-tv-sync	iOS only	-	-
set-geolocation	error	works	-
set-request-interception	error	stub	-
show-keyboard / hide-keyboard	works	works	not supported

Inconsistent error behavior:

• iOS returns PLATFORM_NOT_SUPPORTED for geolocation/interception
• Android returns stub success responses for the same endpoints
• Neither tells the LLM upfront that the command won't work

Proposal

1. Add platforms field to MCP tool definitions: ["ios", "android", "macos"]
2. Add --platform note to CLI help for restricted commands
3. Include platform info in --llm-help output
4. Android stubs should return PLATFORM_NOT_SUPPORTED like iOS (or both should return stub success — pick one, be consistent)
5. get-capabilities response should be surfaced in discovery so LLMs can check before calling

[rafiki270]

Implemented. CLI, MCP, explain, and --llm-help now surface platform support explicitly, discover/get-capabilities expose supported, partial, and unsupported methods, and the iOS, Android, and macOS routers now return PLATFORM_NOT_SUPPORTED for explicitly unsupported commands instead of generic stub errors. That keeps help, discovery, and runtime behavior aligned.