chore: remove stale TODOs for implemented Expand/ExpandVars (bd-qe7j)

- Step.Expand and Step.ExpandVars are implemented in ApplyInlineExpansions
  (expand.go), but had stale TODO comments claiming they were not
- Updated doc comments to reference the implementation
- Updated bd-7zka to reflect which features are still not implemented
  (Step.Condition and Step.Gate remain as future work)

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

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

internal/formula/types.go

@@ -166,12 +166,12 @@ type Step struct {
 	Assignee string `json:"assignee,omitempty"`
 
 	// Expand references an expansion formula to inline here.
-	// When set, this step is replaced by the expansion's steps.
-	// TODO(bd-7zka): Not yet implemented in bd cook. Filed as future work.
+	// When set, this step is replaced by the expansion's template steps.
+	// See ApplyInlineExpansions in expand.go for implementation.
 	Expand string `json:"expand,omitempty"`
 
 	// ExpandVars are variable overrides for the expansion.
-	// TODO(bd-7zka): Not yet implemented in bd cook. Filed as future work.
+	// Merged with the expansion formula's default vars during inline expansion.
 	ExpandVars map[string]string `json:"expand_vars,omitempty"`
 
 	// Condition makes this step optional based on a variable.