chore: add .agents/environment.yaml for automated dev setup

[devin-ai-integration]

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Summary

Adds a .agents/environment.yaml file so that Devin AI sessions can automatically set up a working local development environment for llama.cpp.

The configuration was derived by reading the repo's build docs, inspecting CMakeLists.txt, and performing a full build from scratch on a clean Ubuntu 22.04 VM.

What's included

• initialize: Installs cmake, ccache, libcurl4-openssl-dev, and pre-commit (one-time setup)
• maintenance: Installs pre-commit hooks and runs an incremental CMake build each session
• knowledge: Documents how to lint, test (CTest), build (CPU-only), and run the CLI/server

Review checklist

• Verify the apt package list (cmake ccache libcurl4-openssl-dev) is sufficient for a CPU-only build
• Confirm that running cmake --build build in the maintenance step every session is acceptable (it's incremental, so it's fast on subsequent runs)
• Check whether cd build && ctest --output-on-failure is the preferred way to run tests, vs. ctest --test-dir build --output-on-failure
• Decide if GPU-related build knowledge (CUDA, Metal, etc.) should be added or if CPU-only is appropriate as the default

Notes

• The repo's .pre-commit-config.yaml has a pre-existing dependency conflict: flake8-no-print requires flake8==4.0.1 but the config pins flake8 rev 7.0.0. This is unrelated to this PR.

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Link to Devin session: https://app.devin.ai/sessions/7b2c6e17b066468dbfbfc1d8c1abcc8f
Requested by: @Joseph-Gross

[devin-ai-integration]

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