cmd/sgai/skel: gate workbench MCP on session env

[ucirello]

Summary

• only register the sgai MCP bridge when all session-scoped env vars are present, so workbench config never emits an empty stdio command
• cover the MCP env gating in the Bun plugin tests and assert adhoc prompt actions only export the workspace .sgai config directory
• replace the interrupt flush timeout race in executeAgentProcess with require.Eventually polling so the Go test stops depending on a fixed deadline

Test Plan

• go test ./cmd/sgai -run 'TestExecuteAgentProcessFlushesBufferedTextOnInterrupt|TestBuildPromptActionCommandSpecUsesOnlyWorkspaceConfigDirEnv'
• bun test ./cmd/sgai/skel/.sgai/plugin/workbench.test.ts