Summary

Add token-optimized filtering for cargo nextest output. cargo-nextest is a popular next-generation Rust test runner used as an alternative to cargo test.

Motivation

cargo nextest run produces verbose output with compilation lines, PASS lines for every test, and post-summary recaps. For LLM consumption, only failures and a compact summary are needed.

Proposed Behavior

• rtk cargo nextest [args...] routes through a dedicated filter
• Strip compilation noise (Compiling, Finished, Starting lines)
• Strip all PASS lines
• Show only failure details (FAIL lines, stdout/stderr, backtraces)
• Show a compact summary line (e.g., "301 passed, 0 failed")
• Strip post-summary recap sections
• Verbose mode (-v) shows full output

Expected Token Savings

90%+ reduction on typical test runs (similar to rtk cargo test).

Implementation Notes

• New CargoCommand::Nextest variant in cargo_cmd.rs
• New Nextest subcommand in CargoCommands enum in main.rs
• Reuses the existing run_cargo_filtered infrastructure
• Extracts flush_failure_block helper to DRY up failure block collection