Implement structured output for prompt steps (Phase 1)

Add schema validation and retry logic for AI responses:

- Add output_schema, max_retries, plain_text fields to PromptStep
- Add DEFAULT_OUTPUT_SCHEMA with output/reasoning fields
- Implement append_schema_instructions() for prompt augmentation
- Implement validate_schema() using jsonschema library
- Add retry logic with error feedback on validation failure
- Enhance substitute_variables() for nested field access ({var.field})
- Add jsonschema to dependencies
- Update GUI dialog with plain_text checkbox and max_retries spinner
- Add comprehensive tests for structured output and nested access

Tools using plain_text=True bypass validation (current behavior).
Tools without plain_text get structured JSON output with schema enforcement.

Migration Phase 1 complete. Next: Apply plain_text=True to all 72 tools.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

src/cmdforge/gui/dialogs/step_dialog.py

@@ -6,7 +6,7 @@ from PySide6.QtWidgets import (
     QDialog, QVBoxLayout, QFormLayout, QLineEdit,
     QComboBox, QPushButton, QHBoxLayout, QLabel,
     QPlainTextEdit, QSplitter, QGroupBox, QTextEdit, QMessageBox,
-    QCheckBox
+    QCheckBox, QSpinBox
 )
 from PySide6.QtCore import Qt, QThread, Signal
 
@@ -73,6 +73,31 @@ class PromptStepDialog(QDialog):
         self.strip_fences_check = QCheckBox("Strip markdown code fences from output")
         form.addRow("", self.strip_fences_check)
 
+        # Structured output options
+        form.addRow(QLabel(""))  # Spacer
+        structured_label = QLabel("<b>Structured Output</b>")
+        form.addRow(structured_label)
+
+        # Plain text checkbox (bypass structured output)
+        self.plain_text_check = QCheckBox("Plain text mode (bypass JSON validation)")
+        self.plain_text_check.setToolTip(
+            "When checked, output is returned as-is without JSON parsing.\n"
+            "When unchecked, output must be valid JSON matching a schema."
+        )
+        self.plain_text_check.stateChanged.connect(self._on_plain_text_changed)
+        form.addRow("", self.plain_text_check)
+
+        # Max retries (only visible when structured output is enabled)
+        self.retries_spin = QSpinBox()
+        self.retries_spin.setRange(0, 5)
+        self.retries_spin.setValue(1)
+        self.retries_spin.setToolTip(
+            "Number of retries if JSON validation fails.\n"
+            "The AI will receive error feedback and try again."
+        )
+        self.retries_label = QLabel("Max retries:")
+        form.addRow(self.retries_label, self.retries_spin)
+
         layout.addLayout(form)
 
         # Prompt text
@@ -103,6 +128,12 @@ class PromptStepDialog(QDialog):
 
         layout.addLayout(buttons)
 
+    def _on_plain_text_changed(self, state):
+        """Show/hide structured output options based on plain_text checkbox."""
+        is_plain = bool(state)
+        self.retries_label.setVisible(not is_plain)
+        self.retries_spin.setVisible(not is_plain)
+
     def _load_step(self, step: PromptStep):
         """Load step data into form."""
         # Load name
@@ -125,6 +156,11 @@ class PromptStepDialog(QDialog):
         self.prompt_input.setPlainText(step.prompt)
         self.strip_fences_check.setChecked(step.strip_fences)
 
+        # Structured output fields
+        self.plain_text_check.setChecked(step.plain_text)
+        self.retries_spin.setValue(step.max_retries)
+        self._on_plain_text_changed(step.plain_text)
+
     def _validate_and_accept(self):
         """Validate and accept."""
         prompt = self.prompt_input.toPlainText().strip()
@@ -153,7 +189,9 @@ class PromptStepDialog(QDialog):
             output_var=self.output_input.text().strip(),
             profile=profile,
             name=name,
-            strip_fences=self.strip_fences_check.isChecked()
+            strip_fences=self.strip_fences_check.isChecked(),
+            plain_text=self.plain_text_check.isChecked(),
+            max_retries=self.retries_spin.value()
         )