Summary

Fixes feature flag checking for STDIO and HTTP modes, and adds insiders mode support for HTTP.

Why

Fixes #

What changed

• Uncommented WithFeatureChecker(featureChecker) in STDIO inventory builder
• Added X-MCP-Insiders and X-MCP-Features headers parsing in middleware
• Added new routes /insiders, /readonly/insiders, /x/{toolset}/insiders, and /x/{toolset}/readonly/insiders
• Added ctx helpers WithInsidersMode / IsInsidersMode and WithHeaderFeatures / GetHeaderFeatures
• Updated GetFlags to now read InsidersMode from ctx
• Removed unused Flags field from RequestDeps
• Fixed docs to use "append /insiders" pattern instead of prepend
• Added tests for X-MCP-Features header parsing

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added