Add skill versioning and compatibility

[jonwraymond]

Problem

Skills have no versioning. When tool schemas change:

• Old skills may break silently
• No way to specify tool version requirements
• No migration path for skill updates

Proposal

Add skill versioning:

type Skill struct {
    // ... existing fields
    Version      string                    `json:"version,omitempty"`
    ToolVersions map[string]string         `json:"toolVersions,omitempty"` // toolID -> version constraint
    MinRuntime   string                    `json:"minRuntime,omitempty"`   // Minimum toolcompose version
}

type CompatibilityResult struct {
    Compatible bool
    Issues     []CompatibilityIssue
}

type CompatibilityIssue struct {
    ToolID     string
    Required   string
    Available  string
    Severity   string  // "error" | "warning"
}

func (p *Planner) CheckCompatibility(skill Skill, discovery *discovery.Discovery) CompatibilityResult

Benefit

• Predictable skill behavior
• Safe upgrades
• Clear compatibility requirements

[blocksorg]

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