Implement tofu.commands.search module with searchCommand(): - Case-insensitive substring match on name, summary, version - Sort results alphabetically by name - Output format: zuur/<pool> <name> <ver>\n <summary> - Exit codes: 0 (success), 1 (no match), 6 (network error) - Testable via injectable indexFetcher delegate (7 unit tests) - Wire search dispatch in main.d (replaces stub)