docs: add bgms pull request template

Author: MaartenMarsman

.github/PULL_REQUEST_TEMPLATE.md

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+## Description
+
+Provide a concise summary of the changes.
+
+### Problem / Motivation
+
+If this is a bug fix, describe the specific problem or edge case being
+addressed. If this is a methodological update, explain the theoretical gap or
+need for the change.
+
+If helpful, add a minimal code example that demonstrates the problem.
+
+```r
+# Problem demonstration
+```
+
+### Proposed Changes / New Functionality
+
+Describe the new functionality or logic introduced. Note whether this affects
+prior specifications, sampling algorithms in `src/`, summary outputs, or user
+facing R functions.
+
+Fixes #
+
+If helpful, add a minimal code example that demonstrates the new behaviour.
+
+```r
+# New behaviour
+```
+
+## Type of Change
+
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Breaking change
+- [ ] Documentation update
+- [ ] Performance optimization
+- [ ] Statistical / methodological update
+
+## Documentation and Release Notes
+
+- [ ] Updated function documentation for any signature or behaviour changes
+- [ ] Regenerated `man/*.Rd` files if roxygen comments changed
+- [ ] Updated `_pkgdown.yml` if adding a new exported function
+- [ ] Added or updated `NEWS.md` entry if the change affects users
+
+## Testing and Validation
+
+Describe the checks you ran and the main results.
+
+- [ ] Added or updated tests in `tests/testthat/` when needed
+- [ ] Ran the project styler
+
+```r
+source("inst/styler/bgms_style.R")
+styler::style_pkg(style = bgms_style)
+```
+
+- [ ] Checked test files for styled `result =` captures that must be reverted to `result <-`
+- [ ] Ran `lintr::lint_package()`
+- [ ] Ran `roxygen2::roxygenise()` if roxygen comments changed
+- [ ] Ran `rcmdcheck::rcmdcheck(args = c("--no-manual", "--as-cran"))` for non-trivial changes
+- [ ] Verified numerical behaviour where relevant
+
+## Additional Notes
+
+List any reviewer context that is important for evaluation: open design
+questions, follow-up work, expected numerical differences, or known limits.