feat: support PlatformIO check_tool metadata #645

Context

PlatformIO supports check_tool in platformio.ini for static analysis tools such as clangtidy, but fbuild does not currently expose or document it as a supported compatibility key.

This should be treated as PlatformIO static-analysis metadata, not as part of fbuild's compile path.

Proposal

Add check_tool support to fbuild's PlatformIO compatibility layer.

Suggested behavior:

• Add an optional check_tool field in the relevant config model.
• Parse/preserve/generate check_tool = <tool> for PlatformIO-compatible configs.
• Do not auto-default this field per board.
• Do not consume check_tool during normal fbuild compile/build execution.
• Document that it is only meaningful for pio check / PlatformIO static analysis compatibility.

Acceptance criteria

• fbuild can represent and emit check_tool = clangtidy or another configured tool in generated platformio.ini output.
• No board auto-defaults check_tool.
• The normal fbuild compile path ignores check_tool.
• Documentation lists check_tool as a supported PlatformIO compatibility key and notes it is for pio check, not compilation.
• Tests cover unset, configured, and compile-path-ignored behavior.