github: add Copilot repository-specific instructions

Add repository-specific guidance for GitHub Copilot in
.github/copilot-instructions.md. This file includes information about:
- Coding style (Linux Kernel Coding Style)
- Architectural overview of the project
- PIE code requirements (must be self-contained and depend on compel)
- Descriptions of CRIU commands
- ZDTM test suite details
- Commit message formatting guidelines

This is just initial skeleton designed to optimize GitHub Copilot
reviews.

Signed-off-by: Andrei Vagin <avagin@gmail.com>

.github/copilot-instructions.md

@@ -0,0 +1,95 @@
+# GitHub Copilot Instructions for CRIU
+
+CRIU (Checkpoint/Restore In User-space) is a specialized tool for checkpointing
+and restoring running processes on Linux.
+
+## Coding Style & Conventions
+All C code MUST follow the [Linux Kernel Coding Style](https://www.kernel.org/doc/html/latest/process/coding-style.html).
+
+- **Indentation**: Use hard tabs. Set tab width to 8 characters.
+- **Line Length**: Preferred limit is 80 characters. Max 120 if it
+  significantly improves readability.
+- **Braces**:
+  - Functions: Opening brace on a new line.
+  - Blocks (`if`, `for`, `while`, `switch`): Opening brace on the same line as
+    the statement.
+- **Spaces**: Use spaces around operators (`+`, `-`, `*`, `/`, `%`, `<`, `>`,
+  `=`, etc.).
+- **Naming**: Use descriptive, snake_case names for functions and variables.
+- **Comments**: Use C-style comments (`/* ... */`).
+  - Multi-line format:
+    ```c
+    /*
+     * This is a multi-line
+     * comment.
+     */
+    ```
+
+## Architecture Overview
+- **criu/**: Contains the main logic for checkpoint and restore.
+- **compel/**: Sub-project for "parasite" code injection and PIE blob
+  generation.
+- **images/**: Protobuf descriptions for image files. Use these to understand
+  the state being saved.
+- **restorer**: PIE code that handles the final stages of process restoration.
+  See `criu/include/restorer.h` for `CR_STATE_*` definitions.
+- **crit**: Tooling for inspecting CRIU image files.
+- **soccr**: Library for TCP socket checkpoint/restore.
+- **pie/ directories**: Code in these directories (e.g., `criu/pie/`) should be
+  self-contained Position-Independent Executable (PIE) code. It MUST NOT
+  depend on any external libraries and can only depend on things implemented by
+  Compel.
+
+### CRIU Commands
+- **dump**: Saves a process tree and all its related resources into a
+  collection of image files.
+- **restore**: Restores processes from image files to the same state they were
+  in before the dump.
+- **check**: Checks whether the kernel supports the features needed by CRIU to
+  dump and restore a process tree.
+- **pre-dump**: Performs the pre-dump procedure, creating a snapshot of memory
+  changes since the previous dump/pre-dump (incremental checkpointing).
+- **service**: Launches CRIU in RPC daemon mode, listening for commands over a
+  socket.
+- **dedup**: Starts pagemap data deduplication, minimizing image size by
+  obtaining references from parent images.
+- **page-server**: Launches CRIU in page server mode to send memory pages over
+  the network during migration.
+
+## Development & Testing
+- **ZDTM (Zero-Downtime Migration)**: The primary test suite located in
+  `test/zdtm`.
+- **Test Scope**: Each test case targets a specific kernel primitive type
+  (e.g., file descriptors, sockets, timers).
+- **Test Purpose**: Verifies that the targeted kernel primitive is
+  Checkpointed/Restored (C/R-ed) correctly.
+- **Test Executor**: `test/zdtm.py`.
+- **Running a test**: `sudo ./test/zdtm.py run -t zdtm/static/env00`.
+- **Test Structure**: Tests typically use `test_daemon()` to signal readiness
+  and `test_waitsig()` to wait for the C/R cycle to complete. After being
+  restored, the test checks that all its resources are still in a valid state.
+
+## Commit Message Guidelines
+Follow these principles when forming commits:
+
+- **Separate each logical change into a separate patch**: Each commit must
+  represent a single logical change. Separate bug fixes from performance
+  improvements or API updates.
+- **The commit subject has to start with the sub-system prefix**: Prefix the
+  subject with the affected component (e.g., `criu:`, `compel:`, `images:`,
+  `test:`, or specific file names like `criu-ns:`).
+- **Imperative Mood**: Use the imperative mood in the subject (e.g., "make
+  xyzzy do frotz" instead of "changed xyzzy").
+- **Detailed Body**: Explain the problem being solved (the "why") and the
+  technical details of the implementation (the "how").
+- **Hard Wrap**: The commit message has to be hard wrapped at 72 characters.
+- **Signed-off-by**: Every commit MUST be signed off (`git commit -s`). This
+  certifies the Developer's Certificate of Origin (DCO).
+- **Fixes Tag**:
+  - For bugs: `Fixes: <12-char-commit-id> ("summary")`. The `<commit-id>` has
+    to be the first 12 characters of the commit SHA-1 ID.
+  - For GitHub issues: `Fixes: #<issue-number>`
+- **Atomicity**: Ensure CRIU builds and tests pass after *every* commit in a
+  series to maintain bisectability.
+- **No Fixups**: Squash "fixup!" or "work in progress" commits before final
+  submission.