#8148: docs(install): add build-essential requirement for Linux

## Summary Adds `build-essential` to the system requirements section for Linux users. ## Problem When installing OpenClaw on a fresh Linux system (Debian/Ubuntu), native modules like `sharp` and `node-llama-cpp` fail to compile without build tools. Users see errors like: - `Install Clang or run brew install gcc` - Various `node-gyp` build failures This is a common stumbling block that's easy to miss. ## Solution Added a single line to the system requirements: ``` - **Linux**: `build-essential` for native module compilation (install with `sudo apt install build-essential` on Debian/Ubuntu) ``` ## Testing Confirmed this resolves the issue on a fresh Ubuntu 24.04 server. --- *This PR was created by [Geno-Claw](https://github.com/Geno-Claw) 🦖* <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Updates the install documentation (`docs/install/index.md`) to call out `build-essential` as a Linux system requirement, helping Debian/Ubuntu users avoid native module build failures (e.g., `sharp`, `node-llama-cpp`) on fresh systems. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge; it’s a small docs-only clarification with no behavioral impact. - Only a single documentation line was added, and it aligns with common Linux requirements for node-gyp/native module builds; no code paths or configs are affected. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->