Support additional skill directories via additionalRoots meta parameter

[egor-baranov]

Add support for _meta.additionalRoots in claude-agent-acp.

This change introduces a top-level additionalRoots field in session metadata and forwards it to Claude by merging it into additionalDirectories when creating a session.

Behavior:

• reads _meta.additionalRoots from the ACP session request
• treats it as a string[]
• merges it with any user-provided claudeCode.options.additionalDirectories
• passes the combined list through to Claude unchanged

This keeps additionalRoots as an ACP-level extension rather than a Claude-specific option, while mapping it onto Claude's existing additionalDirectories support.

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[egor-baranov]

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