feat: Make CapabilitySet public

[nemith]

CapabilitySet was a unexported before with the public api being a []string. This exports it and has the session return the CapabilitySet instead of []string exposing the Has() method .

Right now this is a completely frozen/readonly interface which feels right.

[Copilot]

Pull request overview

This PR makes the CapabilitySet type public to provide a more ergonomic API for working with NETCONF capabilities. Instead of returning raw []string slices, the session now returns *CapabilitySet which provides useful methods like Has() for capability checking.

• Exports CapabilitySet type with read-only methods (Has(), Len(), All())
• Renames API methods from ClientCapabilities()/ServerCapabilities() to ClientCaps()/ServerCaps() and changes return type to *CapabilitySet
• Adds constants CapNetConfig10 and CapNetConfig11 for common base capabilities

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
capability.go	Exports CapabilitySet type, adds Len() method, converts All() to return iterator, and adds capability constants
session.go	Updates method signatures to return *CapabilitySet, renames methods to shorter names, uses slices.Collect for iterator conversion
inttest/ssh_test.go	Updates test to use new ServerCaps() method and Len() instead of checking slice length

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