docs: add issue templates, contributing file

Signed-off-by: Slendi <slendi@socopon.com>

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -0,0 +1,59 @@
+name: Bug report
+description: Report a reproducible problem in smath
+title: "bug: "
+labels:
+  - bug
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for filing a bug report.
+        Please provide enough detail for us to reproduce and diagnose the issue.
+
+  - type: textarea
+    id: summary
+    attributes:
+      label: Summary
+      description: What happened?
+      placeholder: Briefly describe the bug.
+    validations:
+      required: true
+
+  - type: textarea
+    id: expected
+    attributes:
+      label: Expected behavior
+      placeholder: Describe what you expected to happen.
+    validations:
+      required: true
+
+  - type: textarea
+    id: repro
+    attributes:
+      label: Minimal reproduction
+      description: Include a minimal code snippet or steps to reproduce.
+      render: cpp
+    validations:
+      required: true
+
+  - type: input
+    id: compiler
+    attributes:
+      label: Compiler and version
+      placeholder: e.g. clang 18.1.8, GCC 14.2, MSVC 19.40
+    validations:
+      required: true
+
+  - type: input
+    id: platform
+    attributes:
+      label: Platform
+      placeholder: e.g. macOS 15 arm64, Ubuntu 24.04 x86_64, Windows 11
+    validations:
+      required: true
+
+  - type: textarea
+    id: additional
+    attributes:
+      label: Additional context
+      description: Add logs, screenshots, or links if relevant.

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1 @@
+blank_issues_enabled: true

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -0,0 +1,41 @@
+name: Feature request
+description: Propose an enhancement to smath
+title: "feat: "
+labels:
+  - enhancement
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for suggesting an improvement.
+        Clear use cases help us evaluate feature requests quickly.
+
+  - type: textarea
+    id: problem
+    attributes:
+      label: Problem statement
+      description: What problem are you trying to solve?
+      placeholder: Describe the limitation or pain point.
+    validations:
+      required: true
+
+  - type: textarea
+    id: proposal
+    attributes:
+      label: Proposed solution
+      description: Describe the API or behavior you want.
+      render: cpp
+    validations:
+      required: true
+
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives considered
+      description: What other approaches did you consider?
+
+  - type: textarea
+    id: impact
+    attributes:
+      label: Impact and compatibility
+      description: Any concerns around performance, API compatibility, or migration?