chore: claude and agents files for contract

[karim-en]

This pull request introduces a new local build target for the smart contract and adds comprehensive documentation for the BTC Light Client Contract. The most significant changes are the addition of a build-local target in the Makefile for easier development on macOS, and the inclusion of a detailed project overview and instructions in contract/CLAUDE.md.

Build system improvements:

• Added a build-local target to the Makefile, allowing developers to build all chain variants locally with macOS-specific LLVM paths. This simplifies development and testing on Apple Silicon machines. [1] [2]

Documentation additions:

• Added contract/CLAUDE.md, which provides a thorough overview of the BTC Light Client Contract, including supported chains, core functions, architecture, block submission flow, chain reorganization, transaction verification, and garbage collection.
• Updated contract/AGENTS.md to reference the new CLAUDE.md documentation file.

[kiseln]

I think all methods are self-explanatory with an exception of gc method

[kiseln]

But gc is also described further down below so I would remove this section completely

[kiseln]

Mentioned below in the same document. Let's keep only one instance

[karim-en]

@olga24912 4414efd

1. Line 5: Changed "A Bitcoin SPV" → "A Bitcoin/Litecoin/Dogecoin/Zcash SPV" to acknowledge all supported chains upfront
2. Removed "Supported chains" and "Core Functions" sections (kiseln noted these are self-explanatory from code and duplicated below)
3. Build section: Added per-chain build commands (make build-bitcoin, etc.) with their feature flags
4. gc_threshold (line 39): Clarified it's mainchain-only, explained when GC triggers (automatically in submit_blocks() + manual), and that fork blocks are unaffected
5. Reorg caveat (line 60): Added warning about GC'd blocks causing PrevBlockNotFound panic during reorg — GC depth must be conservative
6. Garbage Collection section (line 71): Clarified mainchain-only scope and explained batch_size bounds gas usage
7. Feature flags (line 79): Added explicit flag syntax --no-default-features --features ""
8. Zcash (line 86): Fixed averaging window from 45 → 17 blocks (verified from pow_averaging_window: 17 in [network.rs]