fix: return not-found for unknown root commands

[lovewave02]

Summary

• return structured COMMAND_NOT_FOUND for unknown root commands instead of printing help and exiting successfully
• preserve root help for bare raid and existing registered commands
• document the behavior change in the 0.18.0 notes and llms.txt

Closes #125.

Verification

• docker run --rm -v "$PWD":/repo -w /repo golang:1.25 sh -lc '/usr/local/go/bin/gofmt -w src/cmd/raid.go src/cmd/raid_test.go && /usr/local/go/bin/go test ./src/cmd'\n- docker run --rm -v "$PWD":/repo -w /repo golang:1.25 /usr/local/go/bin/go test ./...\n- npm ci\n- npm run build\n- git diff --check\n- binary smoke: /tmp/raid notacommand exits 5\n- binary smoke: /tmp/raid --json notacommand exits 5 with COMMAND_NOT_FOUND\n