chore: add a structured bug report form (#536)

* chore: add structured bug report form

Require version, environment, reproduction, and terminal details so maintainers can distinguish current defects from issues already fixed in newer releases.

Ask reporters to upgrade and reproduce on the latest stable release before submitting a bug.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: update command in bug report template

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* fix: s/textarea/input

Co-authored-by: Sameen Karim <skarim@github.com>

* fix: s/textarea/input

Co-authored-by: Sameen Karim <skarim@github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Sameen Karim <skarim@github.com>

Author: 4 people

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -0,0 +1,115 @@
+name: Bug report
+description: Report a bug or unexpected behavior in gh-stack
+title: "[Bug]: "
+labels:
+  - bug
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thank you for taking the time to report a bug. Please complete the fields below so we can investigate efficiently.
+
+        Before filing, run `gh extension upgrade stack` and confirm the problem still occurs on the latest stable release.
+
+        For security vulnerabilities, follow the instructions in our [security policy](https://github.com/github/gh-stack/security/policy) instead of opening a public issue.
+
+  - type: textarea
+    id: description
+    attributes:
+      label: Describe the bug
+      description: Clearly describe what went wrong.
+      placeholder: What happened?
+    validations:
+      required: true
+
+  - type: input
+    id: affected-command
+    attributes:
+      label: Affected command
+      description: Which gh-stack command triggered the problem?
+      placeholder: gh stack rebase
+    validations:
+      required: true
+
+  - type: input
+    id: gh-stack-version
+    attributes:
+      label: gh-stack version
+      description: Paste the exact output from `gh stack --version`.
+      render: shell
+    validations:
+      required: true
+
+  - type: input
+    id: gh-version
+    attributes:
+      label: GitHub CLI version
+      description: Paste the exact output from `gh --version`.
+      render: shell
+    validations:
+      required: true
+
+  - type: input
+    id: operating-system
+    attributes:
+      label: Operating system
+      description: Include the operating system and version.
+      placeholder: macOS 26.0, Ubuntu 24.04, or Windows 11
+    validations:
+      required: true
+
+  - type: input
+    id: shell
+    attributes:
+      label: Shell
+      description: Which shell were you using?
+      placeholder: zsh, bash, fish, or PowerShell
+    validations:
+      required: true
+
+  - type: textarea
+    id: steps-to-reproduce
+    attributes:
+      label: Steps to reproduce
+      description: List the commands and steps needed to reproduce the problem.
+      placeholder: |
+        1. Run `gh stack ...`
+        2. Select ...
+        3. Observe ...
+    validations:
+      required: true
+
+  - type: textarea
+    id: expected-behavior
+    attributes:
+      label: Expected behavior
+      description: What did you expect to happen?
+    validations:
+      required: true
+
+  - type: textarea
+    id: actual-behavior
+    attributes:
+      label: Actual behavior
+      description: What happened instead?
+    validations:
+      required: true
+
+  - type: textarea
+    id: terminal-output
+    attributes:
+      label: Terminal output
+      description: Paste relevant output or write `N/A`. Redact credentials and other sensitive information.
+      render: shell
+    validations:
+      required: true
+
+  - type: checkboxes
+    id: preflight
+    attributes:
+      label: Preflight checks
+      options:
+        - label: I searched the existing issues for duplicates.
+          required: true
+        - label: I ran `gh extension upgrade stack` and reproduced this issue on the latest stable release.
+          required: true