Add dynamic toolset filtering with expressions

[jonwraymond]

Problem

Toolset filtering in set package is basic. Can't express complex filters like:

• "All file tools except delete"
• "Tools matching regex pattern"
• "Tools with specific annotations"

Proposal

Add expression-based filtering:

type FilterExpr interface {
    Matches(tool model.Tool) bool
}

// Built-in expressions
func NameMatches(pattern string) FilterExpr      // regex
func HasTag(tag string) FilterExpr
func InNamespace(ns string) FilterExpr
func HasAnnotation(key string) FilterExpr
func IsReadOnly() FilterExpr
func IsDestructive() FilterExpr

// Combinators
func And(exprs ...FilterExpr) FilterExpr
func Or(exprs ...FilterExpr) FilterExpr
func Not(expr FilterExpr) FilterExpr

// Usage
filter := And(
    InNamespace("filesystem"),
    Not(HasAnnotation("destructive")),
)
toolset := set.NewToolset(tools).Filter(filter)

Benefit

• Expressive filtering
• Safer tool exposure
• Annotation-aware filtering

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