Capture each server's initialize instructions on the Connection at connect
time and expose them via MCP.instructions(). Add McpGuidance, a system-context
source modeled on SkillGuidance, that renders the <mcp_instructions> block and
hides servers whose contributed tools are all denied for the agent.
instructions() returns just { server, instructions }; the guidance layer
fetches tools() separately and does the agent-aware permission correlation,
mirroring how SkillGuidance composes raw data with agent permissions.
Aiden Cline
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