run: guard empty text part in runAgent + drop cross-repo doc ref (gadfly #16)

Every reviewer flagged that runAgent appended llm.Text(input) unconditionally, so
an image-only run (blank prompt) emitted an empty TextPart — inconsistent with the
sibling runSession.AttachImages which guards it. Mirror that guard
(strings.TrimSpace(input) != ""). Also:
- copy opts before appending (variadic backing array can have spare capacity; avoid
  aliasing a caller's slice).
- reword the doc comment to drop the mort-agentexec reference (executus is a
  standalone lib; a consumer name doesn't belong in its godoc).

Tests: image+text are co-located in ONE user message; an image-only run emits no
blank TextPart.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

run/executor.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"strings"
 	"time"
 
 	"gitea.stevedudenhoeffer.com/steve/majordomo/agent"
@@ -445,16 +446,23 @@ func detach(ctx context.Context) context.Context {
 // input arg, so when the invocation carries images they're folded into the first
 // user message (text + image parts) via WithHistory and Run is called with an
 // empty input — the model then sees a multimodal opening turn. The image-less path
-// passes the prompt straight through. Mirrors mort agentexec's multimodal seeding.
+// passes the prompt straight through.
+//
+// The text part is omitted when input is blank (image-only run), matching
+// runSession.AttachImages so no empty TextPart is sent.
 func runAgent(ctx context.Context, ag *agent.Agent, input string, images []llm.ImagePart, opts ...agent.RunOption) (*agent.Result, error) {
 	if len(images) == 0 {
 		return ag.Run(ctx, input, opts...)
 	}
 	parts := make([]llm.Part, 0, len(images)+1)
-	parts = append(parts, llm.Text(input))
+	if strings.TrimSpace(input) != "" {
+		parts = append(parts, llm.Text(input))
+	}
 	for _, img := range images {
 		parts = append(parts, img)
 	}
-	opts = append(opts, agent.WithHistory([]llm.Message{llm.UserParts(parts...)}))
+	// Copy opts before appending so a caller-supplied backing array is never
+	// mutated/aliased (the variadic slice can have spare capacity).
+	opts = append(opts[:len(opts):len(opts)], agent.WithHistory([]llm.Message{llm.UserParts(parts...)}))
 	return ag.Run(ctx, "", opts...)
 }