docs(claude): add Claude Code project guidance file

[darion-yaphet]

What problem does this PR solve?

Issue Number: N/A

Problem Summary:
New contributors and AI coding tools (Claude Code) have no single onboarding
document when getting started with this repo — understanding how to build the
project and how it is structured requires reading through multiple build scripts
and source directories. This PR adds a CLAUDE.md that consolidates that
scattered information into one place.

What is changed and the side effects?

Changed:

• Add CLAUDE.md, covering:
  • bRPC build commands for all three systems (Make/CMake/Bazel) and how to run a single test
  • Core library architecture overview (brpc/bthread/butil/bvar)
  • Key design patterns: protocol plugins, naming services / load balancers, builtin services
  • Google C++ style with the 4-space indentation convention

Side effects:

• Performance effects: None. Documentation-only change; no compiled code is touched.

• Breaking backward compatibility: None. Pure documentation addition; no APIs,
  build artifacts, or runtime behavior are changed.

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Check List:

• Please make sure your changes are compilable. (Docs only; nothing to compile.)
• When providing us with a new feature, it is best to add related tests. (Not a feature change; no tests needed.)
• Please follow Contributor Covenant Code of Conduct.