Harden Compass CLI help and export contracts

[forhappy]

Summary

• add dedicated, accurate help pages for nested store, program, history-cache, and export commands
• return a nonzero exit status when benchmark execution reports an error
• support atomic --output writes for export callflow-json
• add regression coverage for nested help, benchmark failures, and call-flow JSON file output

Why

Several supported CLI paths either fell through to generic or unknown help, reported benchmark failures as success, or documented a call-flow JSON output path that the parser rejected. These changes make the public CLI contract and exit behavior consistent.

Validation

• RUSTUP_TOOLCHAIN=1.97.1 CARGO_TARGET_DIR=/Volumes/Workspace/crabbuild-target/compass-cli-pr cargo test -p compass-cli --tests --locked
• RUSTUP_TOOLCHAIN=1.97.1 CARGO_TARGET_DIR=/Volumes/Workspace/crabbuild-target/compass-cli-pr cargo clippy -p compass-cli --all-targets --all-features --locked -- -D warnings
• RUSTUP_TOOLCHAIN=1.97.1 cargo fmt --all -- --check
• git diff --check
• sh scripts/check_product_boundary.sh