docs(cli): drop restatement comments in cli tests and helpers

[oferchen]

Summary

Per-crate comment cleanup pass on the cli crate, following the precedent set by PRs #4575 (compress), #4569 (bandwidth), #4584 (filters), #4582 (logging), #4583 (platform), #4587 (matching), #4590 (transfer), #4591 (protocol).

The cli crate is already extensively rustdoc'd (all public items carry docs, #![deny(missing_docs)] is active at the crate root). This pass focuses on the narrow cleanup the precedent PRs target:

• Removed 11 test-scaffolding line comments that merely echo the next statement (e.g. // Create source file directly above std::fs::write(&source, ...), // Verify the xfr line directly above find(|l| l.contains("xfr#")), // Initially all should be false directly above the corresponding assert!s).
• Removed one stale 3-line cluster in filter_behavior_comprehensive.rs whose final line ("Actually for simplicity, just verify the directory is excluded") openly contradicted the preceding two lines.
• Added a leading summary sentence to the rustdoc on frontend::run so the description no longer starts mid-thought ("The function returns ...").

Scope cap

The crate is large (~60 KLoC across 298 files). All // upstream: ... citations, all WHY-annotations (Clap quirks, upstream position invariants, INC_RECURSE semantics, test-section markers, scenario explanations), and all SAFETY-style notes were preserved unchanged. No public-API or test-behaviour changes.

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --no-deps -- -D warnings
• CI matrix (fmt+clippy, nextest, Windows, macOS, Linux musl)