feat: Add --args flag for natural language executor instructions (gt-1py3y)

- Add Args field to SlungWork struct in wisp/types.go
- Add --args/-a flag to gt sling command
- Display args in gt prime autonomous mode
- Display args in gt mol status output

The --args string is stored in the hook and shown to the LLM executor,
which interprets the instructions naturally without schema maintenance.

Example: gt sling beads-release --args "patch release"

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

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

internal/wisp/types.go

@@ -63,6 +63,11 @@ type SlungWork struct {
 
 	// Subject is optional subject line (used in handoff mail).
 	Subject string `json:"subject,omitempty"`
+
+	// Args is optional natural language instructions for the formula executor.
+	// Example: "patch release" or "focus on security issues"
+	// The LLM executor interprets these instructions - no schema needed.
+	Args string `json:"args,omitempty"`
 }
 
 // NewSlungWork creates a new slung work hook file.