fix(formula): address code review findings

Fixes from self-review of formula parser and bd cook:

1. Atomicity: Add cleanup on failure in cookFormula - if labels/deps
   transaction fails, delete the already-created issues

2. Validation: Add recursive depends_on validation for child steps
   and validate priority ranges for children

3. Documentation: Mark unimplemented fields (Expand, ExpandVars,
   Condition, Gate) with TODO(future) comments

4. Thread safety: Add note that Parser is NOT thread-safe

5. Error messages: Track first definition location for duplicate ID
   errors (e.g., "duplicate id at steps[1], first defined at steps[0]")

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/formula/parser_test.go

@@ -198,6 +198,52 @@ func TestValidate_ChildSteps(t *testing.T) {
 	}
 }
 
+func TestValidate_ChildStepsInvalidDependsOn(t *testing.T) {
+	formula := &Formula{
+		Formula: "mol-bad-child-dep",
+		Version: 1,
+		Type:    TypeWorkflow,
+		Steps: []*Step{
+			{
+				ID:    "epic1",
+				Title: "Epic 1",
+				Children: []*Step{
+					{ID: "child1", Title: "Child 1"},
+					{ID: "child2", Title: "Child 2", DependsOn: []string{"nonexistent"}},
+				},
+			},
+		},
+	}
+
+	err := formula.Validate()
+	if err == nil {
+		t.Error("Validate should fail for child depends_on referencing unknown step")
+	}
+}
+
+func TestValidate_ChildStepsInvalidPriority(t *testing.T) {
+	p := 10 // invalid
+	formula := &Formula{
+		Formula: "mol-bad-child-priority",
+		Version: 1,
+		Type:    TypeWorkflow,
+		Steps: []*Step{
+			{
+				ID:    "epic1",
+				Title: "Epic 1",
+				Children: []*Step{
+					{ID: "child1", Title: "Child 1", Priority: &p},
+				},
+			},
+		},
+	}
+
+	err := formula.Validate()
+	if err == nil {
+		t.Error("Validate should fail for child with invalid priority")
+	}
+}
+
 func TestValidate_BondPoints(t *testing.T) {
 	formula := &Formula{
 		Formula: "mol-compose",