[codex] Add Linux kernel indexing

[cla7aye15I4nd]

Summary

• add the upstream Linux kernel using the x86_64 defconfig and LLVM toolchain
• generate and normalize Kbuild compilation commands for SCIP indexing
• install the required Kbuild/LLVM packages
• give every project a measured ccache budget and enable ccache for libyuv
• keep the single-generation compiler-cache budget at 1.6 GB against GitHub's 10 GB repository allowance
• reset ccache counters per build so logs report useful per-run hit rates

Validation

• Linux indexed 3,018 translation units with zero errors
• generated 7,090 source files and 4,077,818 navigable occurrences
• Linux cache restored with a 99.84% hit rate and used about 135 MB of its 500 MB cap
• Linux preview pack is 86,238,751 bytes
• V8 passed with its reduced 100 MB cache cap
• all matrix jobs, preview assembly, pack validation, and source-render smoke tests passed
• parsed every project YAML and syntax-checked every embedded build block
• ran Node, shell, Rust fmt/test/clippy, and diff checks

GitHub Actions cache policy reference: https://docs.github.com/en/actions/reference/workflows-and-actions/dependency-caching