Document Git Core operation contracts

[yangboxuan726]

What changed

• Add the active Git preflight, conflict-marker, and operation-state commands to the Rust Core command table.
• Document their request fields, response fields, ordering, nullable values, and current semantics.
• Add operationContinue, operationAbort, and operationSkip to the git.write operation list and describe their state-aware restrictions.

Why

The shared contract omitted commands and write operations already implemented by rust/lithe-core and consumed by the platform adapters. This update makes the contract match the protocol command enum, Rust request/response types, implementation, and tests without changing runtime behavior.

Validation

• cargo test --manifest-path rust/lithe-core/Cargo.toml git_ (11 unit tests and 3 integration tests passed)
• PowerShell command-name consistency check against rust/lithe-core/src/protocol/command.rs
• git diff --check

./scripts/verify-shared-contracts.sh was not runnable on this Windows host because it requires zsh and /usr/bin/ruby; the only installed WSL distribution is docker-desktop.