feat: extract pkg/ghexpr — deduplicated GitHub Actions expression parsing package

[Copilot]

GitHub Actions expression parsing logic (AST nodes, tokenizer, parser, optimizer, patterns, builders) was spread across 9 files in pkg/workflow with no clear ownership boundary and several obvious pattern duplicates.

New pkg/ghexpr package

Pure expression-parsing infrastructure extracted into a standalone package with full spec documentation:

File	Contents
nodes.go	10 AST node types (ConditionNode, AndNode, OrNode, NotNode, DisjunctionNode, FunctionCallNode, PropertyAccessNode, StringLiteralNode, BooleanLiteralNode, ComparisonNode)
parser.go	Tokenizer, recursive-descent parser, ParseExpression, VisitExpressionTree, BreakLongExpression, StripExpressionWrapper, YAML string helpers
optimizer.go	Boolean-algebra optimizer — 11 rules, fixpoint iteration, status-function safety guard
patterns.go	Core regex patterns + HasExpressionMarker / ContainsExpression / IsExpression predicates
builder.go	Language-level generic builders (BuildAnd, BuildOr, BuildEquals, RenderCondition, …)
doc.go	Package documentation with full GitHub Actions expression grammar spec

Tests: parser_test.go, optimizer_test.go, builder_test.go, patterns_test.go, plus white-box internal tests (optimizer_white_box_test.go, parser_white_box_test.go).

pkg/workflow — backward-compatible delegation

All 38+ existing files in pkg/workflow continue to compile unchanged via Go type aliases and function var aliases:

// expression_nodes.go
type ConditionNode = ghexpr.ConditionNode
type AndNode       = ghexpr.AndNode
// …

// expression_optimizer.go
var OptimizeExpression = ghexpr.OptimizeExpression

expression_patterns.go re-exports core patterns from ghexpr and retains only gh-aw–specific context patterns (AWInputsPattern, SecretsExpressionPattern, etc.).

Duplicate patterns eliminated

File	Removed	Replaced with
expression_syntax_validation.go	stringLiteralRegex, numberLiteralRegex	ghexpr.StringLiteralPattern, ghexpr.NumberLiteralPattern
expression_safety_validation.go	orExpressionPattern (identical to OrPattern)	ghexpr.OrPattern
publish_assets.go	githubExpressionPattern = regexp.MustCompile(...)	ghexpr.IsExpression(value)

Internal white-box tests that directly accessed unexported optimizer/parser symbols are migrated to pkg/ghexpr as package ghexpr (non-_test) files.