Add runnable Prolly S3 branching walkthrough

[forhappy]

Summary

• add a zero-infrastructure, runnable Prolly S3 core walkthrough
• demonstrate commit history, branch divergence, paged structural diff, conflict planning, failed merge safety, explicit conflict resolution, and merged reads
• link the exact command from both S3 READMEs

Why

The S3 client documentation explains each API separately, but it did not provide one small executable flow that connects the repository concepts. This example uses the in-memory versioned object plane so readers can explore the model without AWS credentials or RustFS while exercising the same provider-neutral repository operations.

Verification

• cargo run --manifest-path extensions/s3/Cargo.toml -p prolly-s3-core --example branching_walkthrough
• cargo fmt --manifest-path extensions/s3/Cargo.toml --all -- --check
• cargo clippy --manifest-path extensions/s3/Cargo.toml --workspace --all-targets --all-features -- -D warnings
• cargo test --manifest-path extensions/s3/Cargo.toml --workspace --all-features