Feature/prepare mars2grib for cache

.github/copilot-instructions.md

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+# Copilot Repository Instructions
+
+## Documentation synchronization rule
+
+When working on a pull request:
+
+1. Determine the set of files modified by the PR.
+2. From that set, consider only files under:
+
+   - src/metkit/mars2grib
+   - test/mars2grib
+
+3. For each of those files:
+   - Verify that the related documentation is present
+   - Verify that the documentation is up to date with the code changes
+   - If documentation is missing or outdated, propose the required updates
+
+4. Do not request documentation changes for files outside these paths.
+
+## Definition of “documentation in sync”
+
+Documentation must:
+- Describe the current public behavior and interfaces
+- Reflect any new parameters, options, or outputs
+- Remove references to deleted functionality
+
+## PR review behavior
+
+During PR reviews:
+- Explicitly list the impacted files in the two target directories
+- State whether documentation is:
+  - ✅ in sync
+  - ❌ missing
+  - ❌ outdated
+- Suggest concrete doc patches when needed, referencing the specific lines or sections that require updates.