docs: add guidelines for creating effective GitHub issues (#857)

Author: eleanorjboyd

.github/instructions/issue-format.instructions.md

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+---
+applyTo: '**/github-issues/**'
+---
+
+# Guidelines for Creating Effective GitHub Issues
+
+## Issue Format
+
+When creating GitHub issues, use the following structure to ensure clarity and ease of verification:
+
+### For Bug Reports
+
+1. **Title**: Concise description of the issue (5-10 words)
+
+2. **Problem Statement**:
+
+    - 1-2 sentences describing the issue
+    - Focus on user impact
+    - Use clear, non-technical language when possible
+
+3. **Steps to Verify Fix**:
+    - Numbered list (5-7 steps maximum)
+    - Start each step with an action verb
+    - Include expected observations
+    - Cover both success paths and cancellation/back button scenarios
+
+### For Feature Requests
+
+1. **Title**: Clear description of the requested feature
+
+2. **Need Statement**:
+
+    - 1-2 sentences describing the user need
+    - Explain why this feature would be valuable
+
+3. **Acceptance Criteria**:
+    - Bulleted list of verifiable behaviors
+    - Include how a user would confirm the feature works as expected
+
+## Examples
+
+### Bug Report Example
+
+```
+# Terminal opens prematurely with PET Resolve Environment command
+
+**Problem:** When using "Resolve Environment..." from the Python Environment Tool menu,
+the terminal opens before entering a path, creating a confusing workflow.
+
+**Steps to verify fix:**
+1. Run "Python Environments: Run Python Environment Tool in Terminal" from Command Palette
+2. Select "Resolve Environment..."
+3. Verify no terminal opens yet
+4. Enter a Python path
+5. Verify terminal only appears after path entry
+6. Try canceling at the input step - confirm no terminal appears
+```
+
+### Feature Request Example
+
+```
+# Add back button support to multi-step UI flows
+
+**Problem:** The UI flows for environment creation and Python project setup lack back button
+functionality, forcing users to cancel and restart when they need to change a previous selection.
+
+**Steps to verify implementation:**
+1. Test back button in PET workflow: Run "Python Environments: Run Python Environment Tool in Terminal",
+   select "Resolve Environment...", press back button, confirm it returns to menu
+2. Test back button in VENV creation: Run "Create environment", select VENV, press back button at various steps
+3. Test back button in CONDA creation: Create CONDA environment, use back buttons to navigate between steps
+4. Test back button in Python project flow: Add Python project, verify back functionality in project type selection
+```
+
+## Best Practices
+
+1. **Be concise**: Keep descriptions short but informative
+2. **Use active voice**: "Terminal opens prematurely" rather than "The terminal is opened prematurely"
+3. **Include context**: Mention relevant commands, UI elements, and workflows
+4. **Focus on verification**: Make steps actionable and observable
+5. **Cover edge cases**: Include cancellation paths and error scenarios
+6. **Use formatting**: Bold headings and numbered lists improve readability
+
+Remember that good issues help both developers fixing problems and testers verifying solutions.