refactor(adapters): extract shared session_state_is_current helper

[samzong]

What's changed?

• Extract session_state_is_current into src/adapters/sync_state.rs as a shared helper combining usage and event state checks
• Update Cursor adapter to call the shared helper instead of a local duplicate
• Update OpenCode adapter to use the same helper, replacing inline usage/event checks

Why

• Deduplicate identical incremental-sync skip logic across adapters
• Keep parser-version and include_events handling in one place for consistency

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p recall adapters::cursor (9 passed)
• cargo test -p recall adapters::opencode (13 passed)