[AI SKILL] Add a skill for comparing elf binaries (project-chip#72750)

* Add a skill for comparing binaries

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Review comments

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: Elen777300

.agents/skills/binary-size-comparison/SKILL.md

@@ -0,0 +1,123 @@
+---
+name: binary-size-comparison
+description:
+    Guidelines for comparing binary (ELF) sizes using Rust-based tooling
+    (elf-bloat and asm-annotate) across various platforms to analyze the impact
+    of code changes on footprint.
+---
+
+# Binary Size Comparison Workflow
+
+This skill provides the workflow to compare binary sizes between your current
+branch and a baseline (like the `master` fork-point).
+
+## 1. Prerequisites & Tool Installation
+
+### A. Containerized Build Setup
+
+You MUST use the `podman-vscode-build` skill to handle container setup and build
+execution.
+
+-   **macOS Note**: As per `podman-vscode-build`, use `docker` instead of
+    `podman` on macOS due to stability and proper Rosetta 2 support for the x86
+    toolchains.
+
+### B. Rust Tools Installation (macOS & Linux)
+
+These tools must be installed on your **Host machine**. They require the Rust
+toolchain.
+
+_If you don't have Rust (`cargo`) installed, install it first:_
+
+```bash
+# This official installer works for both macOS and Linux
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
+source "$HOME/.cargo/env"
+```
+
+_Once Rust is available, install the size comparison tools:_
+
+```bash
+cargo install elf_bloat asm-annotate
+```
+
+## 2. Platform ELF Locations
+
+Identify your target's ELF path relative to the repo root before you start:
+
+-   **Linux / POSIX**: `out/<target>/<app-name>`
+-   **ESP32**: `out/<target>/<app-name>.elf`
+-   **Silicon Labs EFR32**: `out/<target>/<app-name>.out`
+-   **nRF Connect (script)**:
+    `examples/<app>/nrfconnect/build/nrfconnect/zephyr/zephyr.elf`
+-   **nRF/Telink (build_examples.py)**: `out/<target>/zephyr/zephyr.elf`
+
+## 3. Comparison Workflow
+
+### A. Build the Baseline (Master Fork-Point)
+
+Determine the fork-point to isolate only your changes.
+
+```bash
+CURRENT_COMMIT=$(git rev-parse HEAD)
+FORK_POINT=$(git merge-base master HEAD)
+
+git checkout $FORK_POINT
+scripts/checkout_submodules.py --shallow --force
+
+# BUILD STEP: Use 'podman-vscode-build' skill here
+# Example: docker exec -w /workspace bld_vscode bash -c "source scripts/activate.sh && scripts/examples/nrfconnect_example.sh all-clusters-app nrf52840dk/nrf52840"
+
+mkdir -p out/size-compares/master-baseline
+cp <PATH_TO_ELF> out/size-compares/master-baseline/baseline.elf
+
+git checkout $CURRENT_COMMIT
+scripts/checkout_submodules.py --shallow --force
+```
+
+### B. Build the PR Branch
+
+```bash
+# BUILD STEP: Use 'podman-vscode-build' skill here
+# Example: docker exec -w /workspace bld_vscode bash -c "source scripts/activate.sh && scripts/examples/nrfconnect_example.sh all-clusters-app nrf52840dk/nrf52840"
+
+mkdir -p out/size-compares/pr-branch
+cp <PATH_TO_ELF> out/size-compares/pr-branch/updated.elf
+```
+
+### C. Run and Analyze Comparison
+
+Run `elf-bloat` on your **Host machine** to generate the report.
+
+```bash
+elf-bloat --compare-base out/size-compares/master-baseline/baseline.elf \
+  --viewer none \
+  out/size-compares/pr-branch/updated.elf \
+  > out/size-compares/size_diff_report.csv
+
+# View top 20 growth contributors (skips CSV header)
+tail -n +2 out/size-compares/size_diff_report.csv | sort -t',' -k3 -rn | head -n 20
+```
+
+### D. Deep Dive Analysis (Optional)
+
+To find the largest functions in a single binary non-interactively (without
+TUI), use `elf-bloat` with the custom viewer to dump a CSV:
+
+```bash
+elf-bloat out/size-compares/pr-branch/updated.elf --viewer custom:cat > out/size-compares/single_size_report.csv
+head -n 20 out/size-compares/single_size_report.csv
+```
+
+If a specific function grew significantly and you want to see exactly which C++
+lines generated the bloated assembly, use `asm-annotate`. It maps the compiled
+assembly instructions back to your original source code.
+
+> **CRITICAL RULE FOR AI AGENTS**: ALWAYS use the `--dump` flag when running
+> `asm-annotate`.
+
+```bash
+# Dump the assembly to a file for analysis or for pasting into a PR description
+asm-annotate out/size-compares/pr-branch/updated.elf "chip::app::CodegenDataModelProvider::InitDataModelForTesting()" --dump > out/size-compares/annotated_function.txt
+cat out/size-compares/annotated_function.txt
+```