fix(pipeline): Use config display value in auto-injection

- Change line 477 in controller.py to use self.config.display or "terminal"
- Previously hardcoded "terminal" ignored config and CLI arguments
- Now auto-injection respects the validated display configuration

fix(app): Add warnings for auto-selected display

- Add warning in pipeline_runner.py when --display not specified
- Add warning in main.py when --pipeline-display not specified
- Both warnings suggest using null display for headless mode

feat(completion): Add bash/zsh/fish completion scripts

- completion/mainline-completion.bash - bash completion
- completion/mainline-completion.zsh - zsh completion
- completion/mainline-completion.fish - fish completion
- Provides completions for --display, --pipeline-source, --pipeline-effects,
  --pipeline-camera, --preset, --theme, --viewport, and other flags

engine/app/main.py

@@ -254,6 +254,16 @@ def run_pipeline_mode_direct():
 
     # Create display using validated display name
     display_name = result.config.display or "terminal"  # Default to terminal if empty
+
+    # Warn if display was auto-selected (not explicitly specified)
+    if not display_name:
+        print(
+            "  \033[38;5;226mWarning: No --pipeline-display specified, using default: terminal\033[0m"
+        )
+        print(
+            "  \033[38;5;245mTip: Use --pipeline-display null for headless mode (useful for testing)\033[0m"
+        )
+
     display = DisplayRegistry.create(display_name)
     if not display:
         print(f"  \033[38;5;196mFailed to create display: {display_name}\033[0m")

engine/app/pipeline_runner.py

@@ -189,10 +189,19 @@ def run_pipeline_mode(preset_name: str = "demo"):
     # CLI --display flag takes priority over preset
     # Check if --display was explicitly provided
     display_name = preset.display
-    if "--display" in sys.argv:
+    display_explicitly_specified = "--display" in sys.argv
+    if display_explicitly_specified:
         idx = sys.argv.index("--display")
         if idx + 1 < len(sys.argv):
             display_name = sys.argv[idx + 1]
+    else:
+        # Warn user that display is falling back to preset default
+        print(
+            f"  \033[38;5;226mWarning: No --display specified, using preset default: {display_name}\033[0m"
+        )
+        print(
+            "  \033[38;5;245mTip: Use --display null for headless mode (useful for testing/capture)\033[0m"
+        )
 
     display = DisplayRegistry.create(display_name)
     if not display and not display_name.startswith("multi"):

engine/pipeline/controller.py

@@ -474,9 +474,10 @@ class Pipeline:
             not self._find_stage_with_capability("display.output")
             and "display" not in self._stages
         ):
-            display = DisplayRegistry.create("terminal")
+            display_name = self.config.display or "terminal"
+            display = DisplayRegistry.create(display_name)
             if display:
-                self.add_stage("display", DisplayStage(display, name="terminal"))
+                self.add_stage("display", DisplayStage(display, name=display_name))
                 injected.append("display")
 
         # Rebuild pipeline if stages were injected