feat: notify dispatcher when polecat work completes

When a crew or other agent dispatches work to a polecat using `gt sling`,
the polecat now tracks who dispatched the work and sends them a completion
notification when running `gt done`.

Changes:
- Add DispatchedBy field to AttachmentFields in beads/fields.go
- Store dispatcher agent ID in bead when slinging (both direct and formula)
- Check for dispatcher in done.go and send WORK_DONE notification to them

This fixes the orchestration issue where crews were left waiting because
polecats only notified the Witness on completion, not the dispatcher.

Fixes: id-c17

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

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

internal/beads/fields.go

@@ -20,6 +20,7 @@ type AttachmentFields struct {
 	AttachedMolecule string // Root issue ID of the attached molecule
 	AttachedAt       string // ISO 8601 timestamp when attached
 	AttachedArgs     string // Natural language args passed via gt sling --args (no-tmux mode)
+	DispatchedBy     string // Agent ID that dispatched this work (for completion notification)
 }
 
 // ParseAttachmentFields extracts attachment fields from an issue's description.
@@ -61,6 +62,9 @@ func ParseAttachmentFields(issue *Issue) *AttachmentFields {
 		case "attached_args", "attached-args", "attachedargs":
 			fields.AttachedArgs = value
 			hasFields = true
+		case "dispatched_by", "dispatched-by", "dispatchedby":
+			fields.DispatchedBy = value
+			hasFields = true
 		}
 	}
 
@@ -88,6 +92,9 @@ func FormatAttachmentFields(fields *AttachmentFields) string {
 	if fields.AttachedArgs != "" {
 		lines = append(lines, "attached_args: "+fields.AttachedArgs)
 	}
+	if fields.DispatchedBy != "" {
+		lines = append(lines, "dispatched_by: "+fields.DispatchedBy)
+	}
 
 	return strings.Join(lines, "\n")
 }
@@ -107,6 +114,9 @@ func SetAttachmentFields(issue *Issue, fields *AttachmentFields) string {
 		"attached_args":     true,
 		"attached-args":     true,
 		"attachedargs":      true,
+		"dispatched_by":     true,
+		"dispatched-by":     true,
+		"dispatchedby":      true,
 	}
 
 	// Collect non-attachment lines from existing description