Add once field for hook handlers (skills context)

[bguidolim]

Phase 1: Once Field

Context

The Claude Code docs define a once field on hook handlers. When true, the hook runs only once per session and is then removed. This is primarily relevant for hooks defined in skills/agents frontmatter.

Field to Add

Field	Type	Description
once	Bool?	If true, runs only once per session then is removed. Skills only, not agents

Files to Modify

• Sources/mcs/Core/Settings.swift — add once to HookEntry
• Manifest/component definitions if pack skills need this

Acceptance Criteria

• HookEntry supports once as an optional Codable field
• Backward compatible with existing settings

Reference

• Hooks reference — Common fields